Abnett does these a lot. In Brothers of the Snake, one of the wildest moments in what is, basically, a series of over the top moments is the defense of an unnamed town on a backwater world. Fifty Space Marines from the Iron Snakes chapter fight a massive horde of orks. There's so many that the Marines know they are going to die- but they do it anyway, because they should try. What's really wild, and what makes this scene so touching, is that they pull it off- for, while all of them are hurt, some quite badly, and the equipment they brought is nothing but worn nubs and blunt spears, not one of them fell- and they killed every single ork. It's one hell of a moment.
Slightly less inspirational, but one of the few times a Space Marine Dreadnought is actually a character, the second excursion to the planet where it all started. A large group of Dark Eldar attempt to hide from the Dreadnought, which can still see them with its sensors. The Dreadnought calls them out on it, then brings down a stone building on top of them. The dreadnought stomps up to one still-living Dark Eldar and demands to know where the Space Marine commander of the mission is. The Dark Eldar spits a curse; the Dreadnought responds "Wrong answer." and proceeds to riddle the entire rubble pile with bolter shells.
In Graham McNeill's novel Storm of Iron, Castellan Vauban, the Imperial Guard commander duels Honsou, the de facto War Captain of an Iron Warriors Grand Company, shatters Honsou's sword, and then cuts his hand off. Considering the IGs are Badass Normals at most, and Chaos Marines are Super Soldiers with daemons in their heads, this is incredibly bad.
It's brought to a sudden halt when Honsou staggers, drawing Vauban into a hasty finishing blow, only to charge inside Vauban's swing and ram the shattered ruin of his sword straight through both Vauban's breastplate and his heart. Ouch.
An even better crowning moment goes to another guardsman, who manages to survive having the rest of the Red Shirts in his guard outpost slaughtered, makes his way back to the base with a Chaos Marine pursuing him the whole way, gets into a missile silo with the instruction of an Adeptus Mechanicus Techpriest, and manages to take out the Marine chasing him, and launch an anti-ship missile into the heart of the Chaos army at the same time.
Also in Storm of Iron, Forrix gets one when he takes out the Warhound Titan that he had been "hunting".
Only to be utterly obliterated by the other Warhound. This was the most delicious moment of the book, considering Forrix was on the cusp of his most magnificent victory.
On the subject of Honsou and company; his sieging of the Indomitable in "Iron Warrior" has to count for something. Not to mention the delicious irony in who M'kar chooses as his host
The Last frakking Chancers. Any and all of them. Constantly.
By the Emperor, the CantiCol Lancers of Henry Zou's novel, Emperor's Mercy. The CantiCol Lancers charge a Chaos War Lord to protect the inquisitor that has been fighting with them knowing full well the consequences of such an action, and they do it anyways. Keep note that said War Lord was jumping off of tank hulls, smashing an FPV from his own weight, and threw a fucking Siegfried super heavy siege tank into the air.
A random Guardsman from the same book gets his own moment as well. During the final battle to stop the forces of Chaos from reaching the Old Kings, he smashes in the faceplate of an Ironclad with the butt of his lasgun, turns and stabs another one in the face with his bayonet, gets shot multiple times, and still manages to strangle a third one to death before succumbing to his wounds.
While both the Fire Warrior game and novel are accused of ridiculous amounts of Beginner's Luck (and rightly so), one genuine moment shines through: Through effectively the second half of the novel, La'Kais has had an unexploded bolter shell (that is to say, a Rocket-propelled grenade) lodged in his helmet after a shootout with a Space Marine. At the end of the incident, Kais is faced with the task of stopping a Daemon Prince alone, with his only edge the chance to weaken his powers by destroying four altars in the chamber. One altar is left. Kais has no weapons, no ammunition, and he just lost an arm. With his remaining arm, he rips off his helmet and throws it. It skids to a stop at the foot of the final altar, where the energy and heat radiating from it cooks that bolter shell off.
"You are not Kharnus Kollidus. You were once something else, something majestic, something transcendent. I pity you. Tell me, before I kill you, what was your name, long ago?" It takes stones to call out a ten-thousand-year-old Reaver-class Titan on using the wrong name.
Albeit he is in a Warlord Titan.
Though in fairness, he had just blown through two more Reavers before attacking that one, and wound up finishing the fight basically unharmed... with three smoking ruins in front of him.
Lau, the Master of Legio Invictia's Skitarii. A gene-bulked cybernetic warrior who dresses like a tribal, hacks enemies apart with his Power Axe, fries them with a plasma pistol grafted to his arm, and rips out their throats with his surgically modified hydraulic jaws. And he goes into battle hopped up on Combat Drugs, yet has the mind of a master tactician.
The Dark Angels combat squad in "Angels of Darkness". While they are outwitted and outmaneuvered by the Falleninfo Basically the Dark Angels Legion was split in two after the Horus Heresy; half of them remained loyal to the Emperor and half of them supposedly fell to Chaos, although it's somewhat implied not all Fallen are Chaos marines, which in fairness wasn't that surprising in that only one of the members of this squad knew about the existence of the Fallen at all, they choose to commit suicide within their own fortress rather than letting the entire planet they're on die of a failsafe-virus in sealed in there with them. They could have easily opened the door and left, condemning the world to doom. In the same novel they single handedly take out what amounts to a small warship. There are 6 of them.
In the same novel, the Fallen Angel Astelan, for his general resilience to the merecies of the Interrogator Chaplain. Put in perspective, beyond the Dark Eldar, the Interrogator Chaplains are possibly they best torturers in the galaxy. He lasts 14 days in total isolation. He's over 10,000 years old too.
Rynn's World, the Space Marine Battles novel, is chock full of these. How about Pedro Kantor beating the absolute shit out of the Arch-Arsonist of Charadon. He beat him up so badly, the Ork Warboss ran away. This is directly after one of the Captains got three of his limbs torn off by the Arsonist's lieutenant in the process of clearing a spaceport in order to save Rynn's World, by allowing reinforcements to arrive.
Helsreach, another Space Marine Battles novel, has it's own fair share. Notable examples include The death of the Godbreaker gargant. Two words: "Engine Kill"
Thunder from Fenris. The battle with the Plague spawn, the end battle and the challenge, "WULFEN!"
Raven's Flight Corax and his remaining Raven Guard fighting the Iron Warriors. You know you're awesome when you have a Powerwhip that can trisect terminator-armoured Space Marines.
Also, the Big Damn Heroes moment at the end, where the Imperial Army and Raven Guard save the rest of the Legion from the World Eaters and Angron
Legion of the Damned, by Rob Sanders, is filled with these. The best is by far one word. Punisher. The little Thunderfire cannon that could. When Khornate Daemons overrun the eastern flank of the city the Excoriators are defending Punisher was the only survivor and continued to hold them off, alone. It held off Khornate Daemons, Cultist Hordes and berserk Chaos Space Marines by itself. And it that wasn't enough when Zachariah Kersh, the protagonist, sends out a signal to fall back Punisher goes on a search-and-destroy mission through the city. Then it blows down a wall, saving the main characters from being overwhelmed by World Eaters and allowing them to escape, before opening fire into the traitors and killing many of them, including 10,000 year old World Eaters veterans who fought at Terra! They finally kill it by flanking it. Punisher died, but it absolutely owned some of the galaxy's fiercest killers before it died.
Then the Legion itself. The Legion of the Damned arrive and absolutely destroy the traitors. Best example is the fate of Umbragg of the Brazen Flesh. By his own admission Umbragg has fought in every single major battle involving the World Eaters. He fought at Istvaan, Terra, Armaggeddon and many other battles. And one lone Damned Legionary comes up behind him and slits his throat, leaving him to bleed out in an alley. One of the galaxy's deadliest warriors died like a common thug in "a street so small and insignificant to the galaxy around it that it didn't even name a name.".
The Legion don't stop there. They fill the city and slaughter the Cholercaust. Some scenes described are a World Eater Lord and his cultists being herded into an open space, only to be fried by the Legion's flamers; A traitor named Slorak the Undying empting an entire clip of ammo from his bolt pistol into the face of a Legionary, only to have his head cut in two; A traitor Techmarine being lured into a park and blown up by pre-placed grenades; Foresworn marines being lured into the catacombs beneath the city and actually being stalked and hunted one-by-one by the Damned Legionaries.
Then there's Scout Omar. During a mission outside the city Omar is ambushed and literally buried alive in a cultist horde after they force him into an open grave. He survives and makes it back to the city as the Cholercaust is siegeing it. Corpus-Captain Kersch rescues him, but Omar loses the lower half of his body to a gelatinous Daemon. That doesn't stop him from being a sniper spotter in the battle, and when his shooter is killed he takes the rifle and continues fighting. He is the last Excoriator to die, the bell tower he is firing on is swarmed by Furies. Even his death is awesome as Omar detonates the grenades he is wearing and takes the tower, and the Fury swarm, with him and the falling tower crushes a hell of a lot of cultists.
And in space... One lone ship is all that defends Certus Minor. They do better than anyone could have expected, taking down many ships with their sole lance cannon. But as they are being boarded a new ship appears. One that is actually visible in from the planet's surface, showing the massive size of the vessel. It then launches a single torpedo and destroys the Keeler Comet that the Cholercaust has been following, the comet was corrupted after passing through the Blood God's realm and had been travelling the galaxy for ten thousand years. And the Raptorous Rex destroyed it with a single torpedo. And then it goes on to annihilate the entire Chaos fleet which consists of hundreds of vessels.
Let's not forget the main character,Corpus-Captain Kersh, The Scourge, future Chapter Master of the Excoriators. He starts the book in stasis, suffering from debilitating nightmares and undergoes invasive brain surgery to stop him from being a drooling vegetable. Now, most of his brothers wouldn't be too bummed about leaving him to rot, so this procedure was only done because he was the only guy in the Chapter who might stand a chance at winning the Feast of Blades, a single combat tournament with no holds barred, and where the occasional death isn't uncommon. That's right, the guy goes from being comatose, straight into a series of brawls, with only a brief procedure bordering on lobotomy to prepare him for it. Obviously, he wins the tournament, reinforcing his reputation for being a ruthless bastard along the way, to the point where the final match is no longer a one-on one fight, but a three way showdown, because none of the other Chapters want to shame themselves by giving him a fair fight. Oh, and he loses an eye in the early matches of the tournament. He's then offered the chance to replace it with bionic enhancements, but turns it down, because he feels the adjustment time might cost him a fight. So he just fills his empty eye socket with a ball bearing.
A subtle and understated one, yet in hindsight one of the most epic moments in 40k literature has to be in Graham Mcneill's The Last Church a story set in the earliest days of the canon in which a man heavily implied to be the very last Christian priest, whom after having his faith verbally ripped to shreds by a mysterious visitor, and having his beloved Church set alight and desecrated by Proto Space Marines, manages to deliver an epic"The Reason You Suck" Speech to none other than the Emperor, beautifully calling him out on his hypocricy and brutality, as well as pointing out with perfect accuracy and foresight how utterly crapsack his vision is, before rejecting him entirely and deciding to burn with his Church rather than be any part of it.
Not sure if it's canon, but some have implied that the Emperor might be Jesus himself. If so, that priest was likely aware that he was flipping off the very figure he had devoted himself to.
The ending of Gunheads. After witnessing the death and suffering of numerous Cadian Regiments - all for a largely pointless suicide mission (in the eyes of the characters) - the reader is treated to this final line:
And men, forever after, would remember this day.
In one of the more recent books, Pariah, the POV protagonist Bequin is being chased by a Word Bearer Astartes. As she's being chased, an old man in an old black long coat emerges from the shadows, and bars the Astartes' way, armed only with an old long sword. He then proceeds to Cut the pursuing Traitor in two, deflect several bolter rounds, and then kill it with a single slash to the head, all while psychically screwing with it. The kicker? It's Eisenhorn, now a couple of hundred years old, and still as unflinchingly badass as he was in Thorn Wishes Talon.
In the novel Blood of Asaheim one of the characters, Baldr Fjolnir, is infected by a Nurglite plague and enters a coma. Meanwhile across the battlefield the Space Wolf Blademaster Váltyr battles against the Chaos Plague Terminator Champioin Thorslax the Blighted and is killed fighting him. Immediately after that the remainder of the Pack attack him, the Wolf Guard Gunnlaugur Skullhewer and his brothers Jorundur the Old Dog and Olgeir Heavy-Hand, and Thorslax dominates them utterly. Just before he can finish Gunnlaugur though Baldr appears, completely corrupted by the plague. Thorslax approaches him and calls him brother, but can't even finish the word before Baldr blasts him with lightning. His armour breaks apart, his boils and blisters explode and his flesh melts; and all the while he is screaming. By the end all that remains of Thorslax the Blighted are a few charred chunks of ceramite and meat. The surrounding army is described as "being unable to decide whether or not they should bow down to Baldr or flee in terror."
Tabletop Game Background Material
Ork Mekboy/biker/Warboss/raving lunatic Wazdakka Gutzmek once found himself up against an Imperial Warlord Titan, a Humongous Mecha protected by powerful energy shields and armed with enough firepower to level entire cities. Undaunted, Wazdakka ramped his bike off a cliff and rammed the Titan with it, overloading the energy shields and setting both him and his bike on fire. The bike continued on its trajectory and slammed into the Titan's head, whereupon Wazdakka, still on fire, proceeded to butcher the Titan's pilot and bridge crew.
Say it with me: WAAAAAAAAAAAAAAAAAAAAAGGGGGGGGHHHHHHHHHHH!
That's not the best part. He kept the skulls of the Titan crew as trophies. To this day, they are still burning.
Hell, even without the sheer awesome of that story, Wazdakka gets a few. For example- he once fitted a scavenged Battle Cannon to his warbike. For reference- a Battle Cannon is the primary armament of a Leman Russ Main Battle Tank, and he had it bolted to his frickin' bike... Quite aside from the inherent ammunition problems of bike mounted tank guns (Battle Cannon shells being almost the size of a human torso); every time it fired the recoil not only slowed or even stopped him, but sent his bike hurtling backwards. Given the average speed and mass of a Speed Freek and his warbike (around 70mph is considered slow, and orks are both physically larger and proportionally more massive than humans, with bikes built in proportion), that implies the sheer scary awesome hugeness of his BFG.
Speaking of Orks, Warlord Grizgutz was prepared to launch his WAAAGH! into the Warp, however, due to a freak Warp Storm he and his WAAGH! travel back in time before they set off. He then attacks his past self in order to have two sets of his favorite gun. The two time-displaced armies then declare war on themselves/each other, inadvertently saving millions of human lives.
Perhaps the best Ork story ever: The Great Boss Tuska fought a demon, decided that the Warp would be fun, and lead a WAAAAGH!, first to Prosan, an Imperial hostile environment training world, for practise, and then into the Eye of Terror itself. While there, they demolished every world they came across, until they met a sentient world, which spawned creatures that were more than a match for the Orks. A Blood Prince came up and slaughtered what was left of the warband. Tuska's Weirdboyz threw pure psychic force at it, and it killed them "with a gesture". With his final breath, Tuska reached between the Blood Prince's legs with his power klaw and "made a gesture of his own". Now, every dawn, the Orks are returned to life, fighting-fit, and are set against opponents they can never beat. In essence, they'd found Ork Valhalla.
Just Orks. They invent a man portable teleportation device, and use it to shoot snotlings (smaller ork sub-species) inside people. They build spaceships so large they have their own gravity well, and shoot asteroids as small arms fire. They use nuclear reactors to power guns (kustom mega-blastas), armour (mega-armour) and just about everything else. In hand to hand they are the equal of a fully equiped space marine in power armour with boltgun (armour that surpasses most modern tanks, armed with a rapid fire grenade launcher) with only an axe and armour that's basically a leather jacket with bits of metal stapled to it. Also see Deffrollaz, Power Klaws and gargants. Oh, and did we mention that all of the above only works BECAUSE THE ORKS BELIEVE IT DOES?
The Ultramarines 1st company defending their chapter fortress from the Tyranids. They managed to hold them back long enough for the Imperial fleet to destroy the Tyranid bioships (by flying a battleship right into middle of them and detonating its warpdrive), but in doing do died to the last man.
Commander Shadowsun had a crowning moment when she managed, by leading several raids, to disrupt a Tyranid splinter fleet so badly that the Tau armada was able to destroy it without losing a SINGLE vessel.
When the World Eaters champion Kharn and his troops were fighting against a warband of Emperor's Children on a planet where during the night the temperatures would drop to lethal levels, even for space marines, he was disgusted because his men stopped fighting in order to escape the cold. So he grabbed a flamethrower and started running around, burning all shelters and slaughtering everybody, friend or foe. He pretty much singlehandedly caused both legions to fragment and earned the name "Betrayer".
Primarchs were made to be awesome, but for this troper a few need to be mentioned here:
Sanguinus for breaking a Bloodthirster's spine on his knee.
He basically held back the entire invading army of Chaos at the gates to the Imperial Palace by himself.
That last part (and damaging Horus' armor in their fight) is the reason that Sanguinus ties with Roboute Guilliman for greatest Primarch in the eyes of the Imperium. And for most fans, Sanguinius beats Guilliman by miles.
Mortarion, for the time before he got found by the Emperor. We're talking about somebody who didn't stop helping people, despite being feared by them and eventually managed to single-handedly drive off invaders from said people armed only with a scythe.
Vulkan squaring off against an attacking force of technologically adept super-sadists with nothing but a pair of hammers. And winning.
Leman Russ is a genetically engineered super Space Viking. This alone makes him pretty awesome, even before mentioning he was Raised by Wolves - Horse-sized wolves with mean streaks a mile wide. The crowning moment, however, would be meeting the Emperor. Russ mocks this stranger, and then invites him to join in the feasting and competition. Russ defeats him in eating and drinking competitions. Let me say that again. Russ beats The Emperor - essentially a meaner version of Jesus - in a drinking competition. The Emperor is pretty pissed off by this point, so he punches Russ while wearing a Power Fist - an anti-tank weapon. After a second, Russ gets up, laughs, and hugs his papa.
He also claimed to have a mild headache after the fight, but caused by the amount of booze he drunk not the punch.
Leman Russ' first act when he emerged from his incubator pod after getting teleported out of the Emperor's laboratory was to climb out of a volcano. Let me repeat that: the first thing Leman Russ did as a newborn child was to CLIMB. OUT. OF. A. VOLCANO.Bad Ass personified.
There´s also the time he broke a glacier with a single punch. A GLACIER.
Nothing compared to the fact that the infant who would one day become Night Haunter to the people of Nostramo, CLIMBED OUT OF THE MOLTEN CORE OF HIS HOMEWORLD.
See Sanguinius's CMOA above? Yeah, Bloodthristers are pretty tough. But Russ? He picked up and broke Magnus the Red. Yeah, the same Magnus that was a Primarch frequently described as a "giant" with a gaze that can kill a man.
Angron apparently managed to slaughter a group of Eldar sent to assassinate him (apparently foreseeing his future as the Khornate Primarch) as an infant.
After the Horus Heresy, all the surviving Traitor Primarchs are content to remain in the Eye of Terror, delegating to their armies or warring against other Chaos forces, and even if they leave they don't do much. Except for Angron, who one day decided to leave the Eye of Terror accompanied with an enormous force of Chaos Marines and daemons, and proceeded to rampage across entire sectors, eventually banished when he was attacked by an entire company of Grey Knights.
Roboute Guilliman vs. Alpharius, no items, final destination. When Fulgrim fought Ferrus Manus, it took a vast number of blows. When Guilliman fought Alpharius, each took only one swing. Guilliman survived. Alpharius didn't. Though there are doubts even within the setting as to whether it actually happened, though if it did it was badass.
And not just for the battle itself: The only reason why the battle happened was because Guilliman broke with Codex Astartes protocol — the very document he had created and claimed to Alpharius was superior to any of the younger primarch's complicated plans and trickery — and directly assaulted the Alpha Legion command camp before deployment finished. In other words, the only way he won that battle was by proving Alpharius right — and he still lost the war for the planet because the Alpha Legion outfoxed him at every other turn.
The way the legion won was also a CMoA. Normally when you beat the leader and slaughter his command structure, the troops go into disarray. Guilliman lost because he was stunned that the Alpha Legion literally didn't care about the death of their primarch. He thought he'd taken out the tactical head of their legion, then he found out why their symbol is a Hydra...
There's also doubt amongst the fandom as to whether or not that the 'Alpharius' slain by Guilliman was the real deal... This is because that the very first thing that any member of the Legion says when they meet someone is "I am Alpharius."
Except it's all lies. The document that the whole account is based on was discovered in the archives of an Inquisitor who was later revealed to be an Alpha Legion spy, and the Ultramarines themselves claim to have no knowledge of it at all. What really happened may well have been awesome for the Ultramarines, for Alpharius, for Guilliman, for the Alphas...or it may never have happened. One theory is that Guilliman actually won a decisive and humiliating victory and the account is the Alpha Legion trying to cover it up. Or, he faked his death. Or he died and won. Or long games are being played by everyone concerned. And this mystery is what makes the franchise awesome in itself.
Night Haunter/Konrad Curze. Even in death he still manages to win. He's left the Imperium and sided with Horus, and assassins have been dispatched to kill him. His reaction? Have his men stand down, sit back, and let the assassin kill him, with the final words "Death is nothing compared to vindication", showing that the Emperor is just as bad, resorting to tactics that are just as heavy-handed to get things done. And that's in addition to being so terrifying that hardened criminals could be sent screaming at the thought that they'd caught his attention, and rogue systems instantly paying every outstanding tithe to the Imperium at the mention that Night Haunter even might be the one to come claim them.
The Sanguinor, implied to be a reincarnation of Sanguinus' noble, idealistic spirit, got a huge one in his first recorded sighting, the Khartas incident. The Blood Angels are chasing some Ork Freebooterz onto a world when it's warp drive opens a rift, allowing Daemons of Khorne to pour through. The Daemons are led by Ka'Bandha, the same asshole Greater Daemon of Khorne who is to Bloodthristers what Bloodthristers are to Gretchin, and fought Sanguinus during the Horus Heresy. Ka'Bandha makes mincemeat of nearly all of the thirty Blood Angels who made planetfall, and the few survivors prepare to make a Last Stand inside a ruined cathedral, when Sanguinor turns up (and by that, I mean he pulls an aerial Dynamic Entry which knocks him out of the sky) and proceeds to utterly own the Greater Daemon, breaking his axe, impaling him through the chest on his sword, and then picking him up, flying into orbit, and body slamming him, reducing the mighty daemon to a giant blood-filled crater (which is still there to this day) and single-handedly fucks up the hordes of lesser Daemons, buying enough time for the survivors to bail.
Marneus Calgar in the Space Marines codex. He charges dozens of Eldar and an Avatar Of Khaine (the Eldar's giant avatar of their god of war) armed only with his powerfists and some super armor. He smashes any Eldar in his way out of his way, attacks the Avatar, grabs its sword when it tries to decapitate him and then smashes it in half with his next blow. Did I mention the avatar is over 20 feet tall and had been smashing away anything in its path for most of the battle with no trouble?
"Only his powerfists?" Come now, the Gauntlets of Ultramar are made of sterner stuff than that!
Canoness Praxedes single-handedly slew a Hive Tyrant in close combat and then led a group of Sisters of Battle in a hit and run campaign against a Tyranid invasion, letting a huge shrine world evacuate through their efforts. This is an ordinary human woman (not a Marine), single-handedly killing a two story tall creature designed to be a close combat killing machine. It is no wonder that she was posthumously given the title of Saint.
Granted, not as inspirational, but Khaine, after being repeatedly wounded and having all his soldiers die, striking down the Nightbringer, who was materialized for about a billionth of a second.
Pre-heresy Iron Warriors, who specialised in garrison and siege tactics. One instance had ten Iron Warriors in a garrison watching over a disgruntled population of 130 million. And they're still pretty badass in the 41st millennium too.
Captain Lysander of the Imperial Fists. After having a streak of bad luck, which culiminated with him getting caught by the Iron Warriors (who hate Imperial Fists), Lysander finally managed to demonstrate his awsome. After suffering a month of horrible torture, he managed to break free of his restrains and escape, armorless and weaponless.
It gets better; once he linked up with the Imperial Fists again, and after a year where they made sure he was uncorrupted, he led his forces back and killed every single Iron Warrior he could find.
Logan Grimnar. After the first war for Armageddon, when any witnesses to the armies of Chaos were shipped off-world to be worked to death, he was the only man in the entire Imperium who had the balls to stare down both the Inquisition and the High Lords of Terra, and call them on their shit, and the only one who could get away with it. Nearly anyone else would have been branded a heretic, nearly any other chapter would have been declared Excommunicate Traitoris. Not him, and not the Space Wolves. Ever since, the Inquisition has walked softly when the Great Wolf is in town. It didn't save the people of Armageddon unfortunately, but that moment of unshakable loyalty to the common men and women of the Imperium, rather then to the heartless machine known as the Administratum, was the closest thing this setting has ever gotten to a Crowning Moment Of Heart Warming.
The master of any first founding Chapter has the authority to do this. No Inquisitor is going to pick a fight without a damn fine reason because they'd essentially be starting a war with that Chapter, their successor Chapters, any Chapters they have solid alliances or a history with and any other Chapters who fear the same thing happening to them later, plus any human forces who ally with the Marines. Any Inquisitor who tried would likely be killed rather than allowed to precipitate a civil war second only to the Horus Heresy.
It doesn't hurt that the Space Wolves are one of the chapters that still maintains legion strength, either. They're known iconoclasts.
Actually, "call them on their shit" is just too lenient (and too weak) for choices of words about his and Wolves' deeds: That period is called as Months of Shame, when Logan Grimnar and Wolves (in honour and trust to those Badass Normal who fought alongside with them during the First War for Armageddon) openly defied the Inquisition's orders to "contain" those who might be tainted by Chaos, by using their own vessels to shield those spaceships (full of survivors from the War) from being fired upon, and eventually, led to direct armed confrontation between the two parties, up to the point that the Inquisition lay siege upon Wolves' homeworld of Fernis. And despite their heavy losses, Grimnar and Wolves stand firm for their ideals. This is both a CMOA and CMOH in itself......Even more so when one considered Grimnar personally killed the two people responsible (one Inquisitor, and one Grey Knights Grand Master) by making their way to their bridge outnumbered (granted at one time he chose to take the bait, but still), and walked away from it unharmed. BAD.ASS.
Grimnar's objections even forced a slight policy change within the Inquisition, something almost unheard of. He has also been known to wage Wars of Compassion against elements of the Imperium itself. This isn't just the actions of a few Space Marines, or a minor Chapter, or even the actions of a powerful Chapter within their own territory. This wasn't the kind of stuff that the Administratum could sweep under the rug or ignore. This was open treason, committed by one of the oldest and most experienced Chapter Masters, one of the oldest and most experienced Chapters, working outside their home territory.
That double bladed axe Logan carries? He got that off a Daemon Prince, one killed on its own homeworld in single-combat. That is more than just a challenge and a half. The weapon then tried, and failed completely, to corrupt him when he picked it up. Let me put that another way, Logan Grimnar is so Badass he made Chaos itself bitch down. Then, because he quite liked the weapon as a trophy, he had it reforged by the Wolf Priests so it wouldn't corrupt those around him.
The 13th Company of the Space Wolves have spent ten thousand years in the middle of a Negative Space Wedgie. Uncorrupted.
In additional to garden variety badassary, each ancient Space Marine Chapter has their own legendary warriors who are each worth an army (or twelve). Ultramarine Chief Librarian Tigurius can mindscrew the Tyranid hordes. Dante of the Blood Angels is prophesied to defend the Emperor at the Time of Ending and can kill daemons in a single blow. Bjorn the Fell-Handed has saved his entire chapter on countless occasions and was killing sorcerers even before he became a Dreadnought. One gets the sense that if/when the Imperium goes under, they're certainly capable of dragging their destroyers down with them.
The Imperial Guard is composed of just regular guys with bad guns (and, admittedly, awesome tanks). They routinely get involved in battles with eight foot tall fungus monsters, undead robots led by the god of death, satanists wearing a solid ton of armor, ravening swarmbeasts who exist solely to eat flesh, and space elves capable of ripping them apart with mind-bullets channeled from hell itself. And they don't always lose.
As a subset of this, a Commissar's job is to be even scarier than all the nasty beasties listed above. They are very good at their jobs.
Ollanius Pius, the guardsman who (before GW retconned him out of existence) faced down Horus himself to give the Emperor time to recover and kick Horus' ass. Having just seen Horus kill a primarch and beat the crap out of the Emperor. With nothing other than his flashlight and cardboard armor. He died, but he held the line for long enough for the Emperor to get it together and end it.
The awesome part is this: he, a lone Imperial Guardsmen, the type who are eaten alive in the millions by enemies across the galaxy, faces down an enemy who just ripped apart a Primarch. He fearlessly steps in between his Emperor and Horus with a flak jacket and a lasgun, while Horus wields a claw bigger than he is. When Horus swats him out of the way like a fly, the Emperor truly realizes he has gone entirely to Chaos, and rends Horus from existence.
1d4 Chan has an interesting account from his point of view, which makes it even more awesome. It ends with this line: Ollanius Pius does the duty his Emperor requires of him. He dies standing, and holding the fucking line.
He's now been retconned back into existance, and made even more Badass. He's a Perpetual now, which means he can't die normally. But nothing more than that, so he's just a Badass Normal with a ton of experience.
The entire Astral Knights chapter. A Necron machine world (imagine the Death Star with hieroglyphs and flashy green Tron Lines) was ripping a new one to a huge human coalition. The Imperials couldn't pierce its shields, all the attempts of teleporting troops on the world failed... So what did the Astral Knights do? They pushed their main battlebarge at its full speed, rammed the machine world's shields so hard they shatter them for a brief moment, and began to destroy as many reactors and shield generators on the surface as possible before being wiped out by the vastly more numerous Necrons. The only Astral Knights who survived were those who were engaged in other battles hundreds of light years away, but those who fought and died made enough damage to the battleship to finally make it drop its shields and offering an easy victory to the Imperium.
Even better, the Imperium allowed the Mechanicum to sift through the wreckage so they could recover the battlebarge, tow it to a desolate world and turn it into a memorial for the entire chapter, with a statue for each of the 772 Battle-Brothers who fought aboard the Necron vessel. And even though the world it's on is a desert wasteland inhabited only by a few scavengers, the memorial is guarded by volunteers from each of the fourteen other chapters who fought beside the Astral Knights that day, including the Ultramarines and the Blood Angels.
Rynn's Might: A Land Raider of the Crimson Fists that, deprived of its crew, fought an Ork warband single-handedly to the end, on behalf of its machine spirits. After running out of ammo and gun barrels, it still tried running the orks over. And once disabled, it opened its hatches, the Warboss blindly stepped in, and...
Slaanesh defeating Khaine, the Eldar God of War and Murder who defeated the Nightbringer, and getting into a fight with Khorne, during which Khaine was accidentally shattered into a million pieces.
Battle of Hades Hive. In which Commisar Sebastian Yarrick stalled the entire Ork WAAAGH! long enough to turn the tide of the war, utilising a severely outnumbered force. During said battle he lost his arm, but stayed concious long enough to kill the offending Ork Warboss, cut off his bionic arm, hold it up as a trophy, urged on his troops, and finished off that attack wave. Not until the skirmish was over did he pass out. Then Yarrick replaced his arm with that of the Warboss. Yeah, CMOA for the Old Man of Armageddon.
And Yarrick has only gotten harder since then. He is probably the only human that can inspire honest-to-Emperor fear in Orks, Ghazghull (the single most powerful and admired Ork in the entire galaxy) considers Yarrick to be a Worthy Opponent, and he is still personally killing Orks despite being several centuries old. When he heard his enemies thought he could kill with a glance, he replaced his eye with a bionic one that could fire a laser specifically so he could do just that.
Nork Deddog was an Ogryn bodyguard to Catachan Colonel Greiss. When Greiss was hurt in combat, Deddog walked over to a downed Chimera vehicle with medicine inside and grabbed it - not the medicine, the Chimera. He then lugged it out of a ditch and twenty meters over to his colonel, showcasing both his badassery and his indomitable loyalty (though not necessarily his intelligence).
It should be noted that through a lifetime of fearless service to the Imperium, Nork managed to do what few of the Imperium's heroes (especially expendable line troops like him) ever accomplish: Lived a full life and died of old age.
One moment of Imperial awesomeness happens when a demon of Slaanesh once tried to rip an entire section of the galaxy out of realspace and into the Warp, his plan being to set up a theocracy with him as its ruler, each major city on every world he ostensibly controlled being host to a giant golden statue in his likeness. When his gambit was about to succeed, he had all of his followers in every city gathered around the statues, worshiping him as he split himself up into enough pieces to be able to fill every statue so that he could feast on the souls of his worshipers. By this point the Imperium had found out and sent a fleet to deal with the place, and at the time where the gambit was about to succeed, they had a ship above every city, targeting every statue with a lance blast. At a predetermined time, every ship opened fire. Every statue was obliterated and every worshiper was killed, annihilating the demon and his followers in one fell swoop.
There's one moment in the Space Marine Battles novel Helsreach, while the commanders of Armageddon's defense are meeting. Commissar Yarrick is talking about how Hades Hive won't survive the week, when a Captain of the Angels of Fire Space Marines Chapter steps up and says that they should try and defend it, because it was where the Ork invasion was broken sixty years ago. Yarrick is adamant in his decision, and the Astartes laughs at him, saying that he's underestimating them and that any Astartes in the room has more military experience than he does. Yarrick doesn't reply, doesn't say anything. He doesn't get angry, doesn't break down. He just stares at the Space Marine, until eventually the arrogant sap realizes he's goofed up and sits back down.
Maugan Ra, Eldar Phoenix Lord of the Dark Reapers, gets a few. First, he was able to defend a planet from a splinter of Leviathan Tyranids, single-handedly, which included bisecting a Tyrannofex. To put that into perspective, the Blood Angels had to call up all of their successor chapters to save their planet from the same fate. Secondly, saving his craftworld, Altansar, which was lost to the Eye of Terror almost ten thousand years ago. During the 13th Black Crusade, Maugan decides to go into the Eye, which at this point of time was launching huge assaults into space, and succeeds in dragging the entire craftworld back out. Here's hoping the other PL's get to do something that badass.
One from the Black Crusade roleplaying game's background; Ax'Senaea the Thrice-Possessed, the Executis Primaris of Laodomida and the sample Daemon Prince, mentioned on the page's entry for "The Baroness", and arguable as a triumphant example of the Badass Normal. In short, this was a woman who so refined her sheer willpower that she was convinced by a sorcerer she could "reverse" a case of Demonic Possession, allowing her to leech off of the daemon's energy to sustain herself instead of being consumed. So she promptly did so... and, to the sorcerer's own surprise (he was expecting her to die), she actually succeeded. Over the course of decades, the daemon trapped inside her soul was painfully drained of power, unable to lift a finger, until she sucked it dry and cast it back into the Warp. Then, as her name suggests, she did it twice more, and digested the next two. This is an awesome moment in itself; whilst some similar cases of symbiotic or human-controlled daemonic possessions get mentioned, it's generally accepted that possession is a Fate Worse Than Death, that leads to the inevitable destruction of the human as their mind is snuffed out and their body becomes a tool. But that's not where Ax'Senaea's awesomeness ends: the very first daemon she captured in this way? A Keeper of Secrets. That's right, the biggest, baddest daemon Slaanesh has in hir army; a monster so terrible that most people lose their souls just looking at it. And it wasn't a fluke; she basically beat down and atethreeEldritch Abominations. She was about to go for number four when Slaanesh hirself showed up and turned her into a Daemon Princess out of a combination of sheer interest in her skills, to make her stop lunching on hir greater daemons, and as a huge "f*** you" to hir greater daemons for not being able to stop her.
One from the new Death Company supplement from Black Library, and perhaps one of the most badass of the Blood Angels to ever live.
During the long years of the Seventh Black Crusade, the full might of the Blood Angels Chapter falls upon a vast Black Legion warband on the world of Mackan. Although the conflict ultimately ends in the near-extinction of the Blood Angels at the hands of Abaddon the Despoiler and his primary lieutenants – the sorcerer-lord Iskandar Khayon and the swordmaster Telemachon Lyras – the Blood Angels Reclusiarch Thalastian Jorus becomes one of the few Imperial heroes to ever land a blow against the Warmaster of Chaos. With his Chapter devastated, the Chaplain endures weeks of hardship in the wilderness and the constant trials of keeping his crazed warriors undetected on Mackan. When the time is right, Jorus leads his Death Company in a lightning raid behind enemy lines, butchering the unprepared sworn warriors of the Despoiler’s honour guard, and allowing the Reclusiarch to lock blades with Abaddon himself. It is said the Warmaster still bears the scars of that battle, even three millennia later. Whatever the truth of the matter, it is known that the Despoiler honoured Jorus once the war was over – perhaps in mockery, or perhaps with nothing but sincerity. After Mackan, thousands of Blood Angels corpses were desecrated, their gene-seed ruined beyond recovery. Of all the Chapter, only a handful of bodies were left undefiled: Reclusiarch Jorus and his Death Company, clad in their battered and broken black ceramite, seated in makeshift thrones made from the armour of those Black Legion warriors they had killed on that fateful night.
When Abaddon the Despoiler gives a guy respect, you know that he's a badass among badasses.
Prince Yriel's backstory. Basically, he was Craftworld Iyanden's highest-ranking admiral, until one day, he led an attack on a Chaos fleet. Although the attack was a big success, he was berated by the Seer council for leaving the craftworld lightly defended. He took his Cool Starship, the Flame of Asuryan, and sailed off, not to be seen again for decades. Later, Iyanden is attacked by the Tyranid hive fleet Kraken, and the Eldar take heavy losses in the face of the swarm. Just as all hope seems lost, out of the blue comes the Flame of Asuryan... followed by ahugeEldar pirate fleet, and they begin turning the tide. Yriel descended to the craftworld himself to take part in the fighting, and took the Spear of Twilight, an immensely powerful weapon that apparently holds the power of a dying star and sucks the soul out of whoever wields it, and used it to personally slay what was implied to have been the Swarmlord. By stabbing it through the face. To re-iterate, the same Swarmlord who is currently cutting vast swathes of destruction through Orkish territory, and once came scarily close to killing the Ultramarines Chapter Master Marneus Calgar. Yriel. Stabbed it. Through the face.
In the Valedor books Yriel topped this, how? By wounding the Hive Mind itself.
The New Damnos War Zone book is just full of Ultramarines being Crazy Awesome, long story short the 2nd Company failed to save the world form Necrons so the Ultramarines lead a force to win it back. Some things included, Calgar lifting a Necron Pylon and firing it at the Necron battleline. Scouts sneeking into the Overlord's chamber and panting melta bombs so when he returns for repairs it blows up in his face. and captain Sicarius of the 2nd killing a transcendent c'tan (one of these gave the Emperor a hard time) in a duel by jumping on top of a body of the Necron Lord he was fight to reach it.
The fluff for Commander Farsight and his True Companions, collectively known as The Eight. A world under Farsight's protection was being nommed by Tyranids, and with his defenses rapidly being overrun, Farsight had the planet evacuated while he and his allies retreated to a top-secret, secure lab to protect the last few Earth Caste scientists working on a way to fight the Tyranids. Though he'd done all he could to hide the lab, it was eventually discovered by the swarm. What followed was a Hold the Line of true awesome, eight battlesuit-equipped Tau (six Crisis Suits, a Broadside and a Riptide) against an entire swarm of Tyranids. Farsight himself duelled the fuckingSwarmlord, keep in mind, the same Swarmlord who almost killed Marneus Calgar, with just his Dawn Blade against all of its swords. The other members of the Eight cut down a massive number of Tyranids until eventually the Air Caste came to extract them, retrieving The Eight plus one of the Earth Caste scientists (who in a badass move of his own took control of the Riptide Battlesuit and used it against the Tyranids, Farsight declared him an honorary Fire Caste member for this). At first it seemed like it had all been for nothing, as the Tyranids nommed the planet's biomass as usual. Then the hive fleet began to rot and decay until it was completely annihilated, and it turned out that the Earth Caste scientists had developed a biotoxin which would take effect once assimilated by the hive fleet, and sacrificed themselves to ensure it would reach the fleet.
Deff Skwadron. Back to front, cover to cover awesome. I mean, you've got Orks, and planes, and dakka, and putting those together automatically results in awesome, but Deff Skwadron just takes that awesome and ramps it up twelve times.
One of the issues' plots involves Deff Skwadron finding out their front lines are under attack, while their Fighta-Bommas are all in the middle of being repaired. After the Skwad commander guns down all the Meks repairing the Bommas (on grounds of suspected sabotage) they then proceed to chain the disassembled Bommas' engines to their Wartrukks and Warbikes and charge into battle. Let me say that again: they drove jet-propelled motorcycles into combat.
Heh, good ol' Killboy...
How about the issue "Sink Da Grimlug" where they're sent to take out a massive enemy battleship, get shot down, take the ship out from the inside, and fly away on a stolen Bommer... Which they decide to use to bomb the base of a rival skwadron.
Killboy wading into the ravenous Squig payload to open the bombing bay from the inside, because he's the only one with a cybork body. He survives the bombing, the landing, and the ensuing panic.
The Redeemer. A batshit crazy Priest from Necromunda. Picture Batman with his head on fire, a chainsaw sword, snappy one liners and henchmen who sing a modified version of the Battle Hymn of the Republic in his honor as he kills the alien, mutant, the heretic. Yeah, it's good. "IF IT DOESN'T HURT! IT DOESN'T COUNT!"
Behind The Scenes
Deathwatch is stated, by the developers, as encouraging these, with a link to the tropes page. That's somewhere.
Tabletop Game Battle Reports
The ork warboss known appropriately as Killaboss had not one, but TWO massive CMOAs in the most recent White Dwarf's apocalypse battle report. His first was taking down a warhound scout titan with what was described as a "classic right haymaker against the somewhat higher chin of the titan". Killaboss then survived a DIRECT HIT from a Kill Sat fired at him by the warboss whose position he was trying to usurp, and was only taken down when a good deal of the combined remaining imperial forces, including super-heavy tanks all opened fire on him.
The Storms Of Ixia Narrative Campaign features numerous examples of these:
In the very first mission, an Orcish horde under Warboss Gruffnod and his lieutenant Big Mek Grotchucka are attempting to overrun an Imperial Guard encampment. One of the Imperial Guard officers is a Master Of Ordinance, who can call down a lance strike from the orbiting frigate every turn. However, due to Grotchucka's "Disrupshun Device", there's a 50% chance that the Ork player will get to place the lance strike instead. On the first turn, the Master Of Ordinance chooses to take the gamble and call in a strike. He succeeds in getting the instructions through, and scores a direct hit on Grotchucka and his Mek squad, completely vaporizing Grotchucka.
A Striking Scorpion squad attacks the Imperial Guard lines. All of them save the Exarch are cut down by massive amounts of lasgun fire. The Exarch runs into a guardsmen squad, and massacres three guardsmen with his claw. The guardsmen hit back for the Exarch to laugh them all off. Next turn, the Exarch kills one of the guardsmen, and again survives being assaulted by the rest of the squad. The Guardsmen, now panicking, try to flee but fail to get away and the Exarch kills them all in retreat. That Exarch then runs into a heavy weapons squad and begins chopping them up as well, massacring all save the Sergeant and the Lord Commissar; the Lord Commissar, enraged by the perceived incompetence of his men, executes the Sergeant and challenges the Exarch alone (that in itself is pretty badass by the way). The Lord Commissar takes a mandiblaster hit to the chest, and then three melee attacks with the scorpion's claw, which all hit and wound. The Exarch is then cut down by Imperial Guard shooting, but... Damn...
In this fan video, the scene where the Dreadnought comes out. The scene where they first meet the genestealers could also count.
In this there are at least two or three every minute. Especially when the Dreadnought from Dawn of War 2 breaks through the wall and kills the Eldar, and when the Blood Raven (wearing Force Commander armor, but has Tarkus' face) beats down an Ork Warboss with his Thunder Hammer. Heck, just the music alone is badass enough.
The Tvtropes entry for Everything Trying to Kill You once contained a Crowning Moment for all of 40k. To quote: "If the scenery is trying to kill you, that's a case of Malevolent Architecture. If the entire planet is trying to kill you, you are on the dreaded Death World. If the entire universe is trying to kill you, you're fucked." The phrase "you're fucked" contained a link directly to the Warhammer 40000 page. Sadly, the page appears to have been edited.
One White Dwarf issue a few years back featured someone who had built a diorama of a Tau Stealth Suit (page 2 of this PDF) which blends in so well with the backdrop from the right angle that you can barely see it. Bear in mind that the backdrop isn't just some vague shape; it's a pretty heavily detailed Imperial-made wall, with panels, grilles and even skulls. Now that is stealth.