The Tanith First and Only a.k.a the Gaunts Ghosts. As Light Infantry they're even squishier than other Guard regiments, yet that doesn't stop them from pulling off feats that would get other Imperial Guard units annihilated. Most of these feats consist of winning against vastly superior numbers of Chaos forces. Individual characters and squads have been known to kill all sorts of things that could easily wipe away enemies and units that could potentially destroy entire armies of Imperial Guardsmen all by themselves. These include, but are not limited to: numerous Chaos warlords, a Dark Eldar assassin, high-level psykers, a Chaos Dreadnought, and a squad of Chaos Space Marines.
Scout Sergeant Oan Mkoll is hardcore even by Tanith standards. That Chaos Dreadnought the Ghosts killed? A team effort helped by a lucky slash from a power sword and judicious use of flamers. In Ghostmaker, Mkoll kills a Dreadnought by himself. The rest of the regiment doesn't know about this incident, and they still believe him to be completely invincible and theStealth Expert.
Ciaphas CainHERO OF THE IMPERIUM, who has personally and successfully dueled everything from psykers and Greater Daemons of Slaanesh, an Ork Warboss, Khornate Berserkers, and Tyranid Hive Tyrants, Broodlords, and Genestealers, along with countless mooks and minions of all of the above. He certainly gets a lot of help from Jurgen and his trusty meltagun, but he killed the Warboss and a Berserker in single combat and all he had was a laspistol and chainsword. He's described by Vail to be one of the best marksmen and duelists she's ever known. Coming from an Inquisitor, that's extremely high praise.
In the related video game, La'Kais probably qualifies as well. Sure he's an alien, but he's a Tau, which means he's shorter lived and physically weaker than even the average human. And yet his first day of live fire combat he personally killed several battalions of the Imperial Guard, large numbers of Space Marines and several Dreadnaughts, a good deal of Chaos Marines and several Daemons, and he does it all within the timespan of twenty four hours. Alas, he breaks down after that.
The Tau in general managed to survive an Imperial crusade, a Tyranid splinter fleet, defeated an Ork WAAAGH! and supposedly killed a daemon prince. The last in particular is quite an accomplishment since killing a daemon prince without the proper warp killing tools at hand is still an admirable effort.
The Horus Heresy novels also contain a fair amount of them. The one that stands out particularly was the mute bodyguard Maggrad, who was by the standards of ANY universe containing Space Marines, an extremely proficient killer. This was further emphasized later when he was able to almost best an extremely experienced Space Marine captain. Dinas Chayne, another bodyguard, was also an incredibly talented soldier who was, even more amazingly, capable of briefly matching a Primarch in term of swordplay.
Maggrad, having fought waves of mutants to a standstill to protect his charge, is actually congratulated personally by Horus. The giant, superhuman, galaxy-bestriding Primarch and Emperor's favourite who he practically worships actually tells Maggrad that he admires him. Cue the closest thing to Squee in Maggrad's life.
Guardsman Hawke. In Storm of Iron, he escapes the initial assault (blowing away a couple of Chaos Space Marines with an assault cannon as he does), hoofs it to an orbital torpedo silo, and takes out half of the Iron Warriors Chaos Space Marines besieging the Citadel. He's also the only Imperial Guardsman to survive the whole war, not counting slaves hauled off to Medrengard (though a few Imperial Fists in a Thunderbird manage to save him, whether or not they were in the campaign or just happened to be investigating is unknown). To put that in perspective, he had been written off by his superiors as a useless, foul-mouthed, insubordinate and ill-disciplined Guardsman barely worthy of the name prior to the attack.
Catachan Guardsmen are a level of badass most other Guardsmen can only dream of. Their homeworld is a Death World that eats other Death Worlds for breakfast, where surviving to the age of ten is considered an accomplishment equivalent to graduating from boot camp, and the ones sent to the Guard are the best of the PDF forces. Possibly somewhat expected, the average Catachan Guardsman is basically Rambo.
Eldar Guardians. Guardians are strictly a civilian militia used to supplement and support the more well-equipped and professional Aspect Warriors, as well as serve as vehicle and artillery crews. However, even these unassuming weekend warriors are amazing fighters by human standards: tellingly, in Necromunda, they're treated as Elite Mooks.
Any high point GURPS Character will be this, if they didn't load up on lots of fancy exotic advantages.
Any high-level martial character in any edition of Dungeons & Dragons. Fighters? Yes. Warlords/Marshals? Yes. Rangers? Yes. Barbarians? Yes. Rogues? Yes. Barbarian/Fighter/Frenzied Berserker? OHSHI—
In 4.0 there is an epic destiny for Rangers and Rogues called Godhunter. So yes, your halfling armed with shurikens and a dagger may in fact one day murder Tiamat.
In 4th edition, there are 14 Epic Destinies, counting the Godhunter, restricted to Martial characters alone (that is, Fighters, Rangers, Rogues and Warlords — Barbarians are Primal in 4e). The Adamantine Soldier is a Fighter or Warlord who has become such a master of heavy armor that they are Nigh Invulnerable; nothing hurts them, nothing can get past them, and they will never be broken in body or spirit. The Beastlord is a beastmaster Ranger who becomes so close to his animal companion that they become a single mind and soul. The Dark Wanderer is a Rogue or Ranger who has wandered so long and so far that they have outrun fate itself. The Eternal Defender is a Fighter who becomes an immovable object in battle, whose courage and duty leads to them taking up arms to protect the world or even The Multiverse for all eternity. The Legendary General is a Warlord who goes on to fight in the greatest battles the planes have ever seen, and if they so desire, may conquer all of reality itself. The Martial Archetype is a multiclassed Martial character who bcomes the ultimate warrior. The Perfect Assassin is a Rogue so skilled at dealing death that even Death itself will not dare to approach them before the Perfect Assassin is ready to die — they may even slay the God of Death and take the throne for themselves. The Undying Warrior is the ultimate Fighter, immortal and unkillable. The Warmaster is a Warlord who becomes so skilled that there is no challenge left in reality — defeating armies of demons or outwitting the gods themselves becomes trite, forcing them to seek the Eternal War, the mythical arena where all of the greatest military leaders who have ever lived spend eternity testing their skills against each other. The Dragonheart is a Martial character who reveres dragons as the ultimate embodiment of martial virtue, devoting themselves to those concepts with such fervors that they transcend their original nature and become dragons in body and soul. The Invincible Vanguard is a Martial character is a warrior who yearns to face the greatest threats ever, and who goes on to defeat them all. The Legendary Sovereign is a Fighter, Ranger or Warlord who goes on to found a kingdom, or perhaps an empire, that will never be forgotten. And the Star-Favored Champion is destined to become, simply, one of the greatest heroes ever known.
In d20 Modern Urban Arcana, a high level hero without special powers can be this. Despite possessing no surnatural abilities, a Fast Hero5 Gunslinger 10 Sniper 5 is a force to be reckoned with, able to bring down demons with nothing more than oneBarrett M82 shot from more than half a kilometer away, dodge grenades (Evasion) and bullets like there was no tomorrow (Huge class Defense and Dexterity), even if he/she didn't see the shooter, because he/she just had a gut feeling (Uncanny dodge), cannot be flanked (Improved Uncanny Dodge), can use stealth to get the jump on almost anything, spend time to aim an enemy and ignore cover less than one half (Improved Dead Aim), lay down a barrage of fire (lightning shot), is almost invincible when taking cover (Defensive Position)...
The Sourcebook "Mirrors" introduces the Extraordinary Mortals template, which gives characters access to "Skill Tricks", specialized abilities within their mundane Skills. The best of these verge on Charles Atlas Superpower.
Any of the non-super OOC from Rifts, but in particular the Rogue Scholar. For people whose sole class ability is "Knowledge is Power", they sure do have quite a reputation with the Coalition. And by reputation I mean "KILL HIM NOW! Now, HE'S the biggest threat!"
Iron Heroes thrives on this, with 9 out of 10 classes having no magical talent at all.
This is the whole purpose of the Guild in Exalted. In a World ruled by beings so powerful that their original job description was beating up the gods themselves, the Guild's purpose is to provide Puny Humans with a power base of their own. They are so successful that the Guild, rather than the Realm, is the only true power with influence across the whole of Creation, with the wherewithal to bring entire countries to their knees if displeased.
The Burn Legend ruleset allows normal people with martial arts training to compete, albeit at a disadvantage, against element benders, shapeshifters, and guys with the power of the Thousand Hells surging through their veins. With a few good clash rolls and some decent timing, they can even win.
Corvus Belli's Infinity has this in the form of the Ariadna faction. Lacking the technological advantages of their rivals, the nations of the planet Dawn make due with guerilla tactics, a mean-streak in CQC and an ungodly amount of firepower. Also, this makes them immune to the hackers of Pan O or the Nomads* . The one character this troper can think of who isn't immune requires you to be within CQC range. That character? William Wallace..