Spartacus slaying Theokoles and becoming The Bringer of Rain.
Before that, Theokoles pretending to die to lure Spartacus and Crixus into a false sense of security.
Theokoles: Capua, shall I begin!?
Spartacus chokeslamming Gnaeus off the cliff to his death after Gnaeus mocks Pietros' suicide.
Ilithyia killing Licinia by grabbing her by the throat and bashing her head against the floor nine times, which reduces it to pulp. "Stop laughing!"
Spartacus and Varro are fighting opponents while chained together and have flawless teamwork. Their final foe is a warrior covered from the neck down with heavy armor, so they wrap their chain around his neck and pull until he's decapitated.
Calvinius: You are beneath me, and you always will be, in this life or the next!
Crixus returning to the arena and defeating Pericles by stomping his head to pulp.
Solonius putting up one heck of a fight against Spartacus despite his old age. Right before he dies, he bursts into laughter. Because Spartacus agrees with him that the wrong villain is being executed this day, and assures him that Batiatus will join him soon. According to Word of God, Solonius was originally planned to die like a coward by trying to flee - but the actor's good physique led to them giving him this moment.
Spartacus defeating Glaber's guards while armed with a wooden sword.
Mira gets a downplayed one in "Whore". Lucretia essentially sends her to be raped by Spartacus and threatens her with A Fate Worse Than Death if he refuses her again. She cuts a deal with Spartacus instead, and when Lucretia asks if he accepted her - Mira simply responds "he made many demands of me", already establishing that she has a duplicitous side.
The entire Season 1 finale easily qualifies.
Mira killing a guard with a pot shard. Reminder that Mira was the member of the rebellion to start it all, and she did it with fucking flowers in her hair.
Crixus launching Spartacus into the balcony, kick-starting the Gladiator Revolt.
Crixus: "Kill them! KILL THEM ALL!!!"
Ilithyia leaving Lucretia and the others behind and locking the front door, preventing them from escaping the gladiators, as revenge for all the crap she's been put through.
Aurelia getting revenge for her husband Varro's murder.
Aurelia: Was it yours? The small little voice? Did it form the words that robbed Varro of life?
Numerius: Varro?!? He deserved to die. He was nothing...
Aurelia: What did you know of the man? That he would sing to his child, sleeping in his arms? Or kiss the tear from my cheek, when I was of a mood? Did you know the father? The husband? (she caresses Numerius's face) He was not a perfect man. But he was mine. He was MINE!!!! (she stabs him repeatedly with a knife) HE WAS MINE!!! HE WAS MINE!!! HE WAS MINE!!! MINE!!! MINE!!!!
This scene highlights, for any who still had doubt, the systematic, institutionalized evil of slavery.
Batiatus killing a few of the gladiators.
Spartacus killing Batiatus and Crixus nearly killing Lucretia.
Spartacus' final speech to the newly freed gladiators and slaves.
Spartacus: I have done this thing because it is just. Blood demands blood. We have lived and lost at the whims of our masters for too long. I would not have it so. I would not see the passing of a brother, for the purpose of sport. I would not see another heart ripped from chest, or breath forfeit for no cause. I know not all of you wish this, yet it is done. It is done. Your lives are your own. Forge your own path, or join with us, and together we shall see Rome tremble!
Gods of the Arena
Gannicus shrugs off getting stabbed in the chest with a huge nail and defeats an opponent while blindfolded.
Crixus facing Gannicus evenly despite being a rookie at this point.
Crixus defeating Auctus and finally gaining respect in the House of Batiatus, becoming the badass we know and love.
Titus trying to choke Lucretia despite being on his deathbed when she reveals that she's the one who's been poisoning him.
The ploy to capture Tullius, ending with Batiatus burying him alive.
Gnaeus setting his net on fire and throwing it on an enemy.
Gannicus slaying Caburus. Caburus has been No Selling every attack used on him, so Gannicus puts a spear through his mouth and pulls until his lower jaw comes off.
Spartacus punching Agron and calling him out on lying about Naevia.
Ashur easily killing two of the rebels.
The captured Gaul scaring the crap out of Seppia with a Death Glare. Ilithyia gets annoyed at her and strikes him down without hesitation. Later, Ilithyia punches Lucretia down, grabs her by the throat, and threatens to kill her the same way she killed Licinia.
Nasir charging into battle despite not being a trained warrior and actually doing damage.
Episode 6 "Chosen Path" is one huge one for Ashur. He finally gets Glaber and his men to listen to his warnings not to underestimate the rebels by slaughtering Glaber's soldiers until they're too scared to approach him, then pointing out he was the weakest of the gladiators. He calls Lucretia out on her bullshit, then travels and collects savage mercenaries to assemble.
Glaber rightfully calling Varinius a Dirty Coward as he fled after the arena collapsed without helping them at all.
Naevia ordering Crixus to teach her how to fight so she will never be raped again.
Spartacus vs Gannicus. The sky is pitch black and both combatants interchange with dual blades and hand-to-hand combat.
Mira shooting the traitorous Chadara with an arrow.
Glaber killing Seppius. He chokes his throat by pressing down his foot until his eyes fill with blood.
After liberating a new batch of slaves to recruit, one particularly huge German named Sedullus caused a few problems for the gang. Nothing major at first, he potentially revealed their position on a raid, went hunting without Spartacus' permission. All problems that can be fixed with a little discipline. Then he tries to rape Naevia, starts a riot, and attempts to kill Agron, probably the only guy defending him. What does Spartacus do? Slice his face clean off. He then tells the Germans to join him, leave, or die. They join him, and every rebel pounds their chests in unison.
Ashur makes Lucretia call him "Dominus".
Gannicus choosing his side, most likely out of revenge for Glaber crucifying that prostitute he was fond of, by slaughtering Ilithyia's escort and kidnapping her. He punctuates this by leaving his rudis lodged in the throat of the mook who was tailing him. The message is clear: he is a rebel.
Lucius marching into Glaber's base to deliver Spartacus' terms and calling him a cunt. That took some serious balls.
After several episodes of being the last surviving man to give Glaber shit, a catapult round melts Varinius's smug face away. Sure they were aiming at the temple Spartacus and co. were hiding in, but Glaber had to have had a little victorious laugh when he saw the charred remains.
And then there's Ilithyia killing Seppia to save Glaber's life — cutting her throat so the blood sprays across her naked husband, in a complete reversal of the traditional damsel in distress scenario.
Lucretia outsmarting Ashur and effectively destroying him when he thought he'd won, using just a few choice words in the right ears and Seppius' bracelet. When Ashur realises (far too late) what's happened, it's a big Oh, Crap! from him and serious Catharsis Factor for everyone else waiting to see the smug bastard trip himself up. It then leads to...
....Naevia gelding and then decapitating Ashur. So many people had reason to want that guy dead, so it's nice that they managed to work out who should have priority.
Spartacus, Crixus, Gannicus, and Agron climbing down the mountain on vines and taking the Roman guards by surprise. They then steal the catapults and launch fireballs at the Roman camp.
Spartacus: I know fear swells within your breast, and it does so with great cause. The Romans hold numbers far outweighing our own; yet they have revealed, time and again fatal flaw. That they believe: the Roman way, is the only one. Just as they believe there is only one way to leave this mountain. We shall teach them that we forge our own paths, and nothing in this world is impossible-when heart and mind are put towards it... Let us begin a lesson forever remembered... we are all Gods this night, and the Romans, WILL FEEL OUR WRATH!
Spartacus briefly fights the Egyptian, who up until then has bested everyone including Crixus and Oenomaus. The battle is very short and one sided with Spartacus forcing the Egyptian to retreat.
Oenomaus cutting the Egyptian across the abdomen and then punching the wound with enough force to stagger the Egyptian. Then he says "I owe you pain!"
Gannicus caving The Egyptian's skull in. Especially the scene before when he attempt a roundhouse kick that gets caught and then follow with the his other leg which knocks the Egyptian to the ground.
On the verge of death, Glaber boasts that Rome will send legions in his wake. Spartacus says they'll just kill them too and rams his sword down Glaber's throat.
The fact that Lucretia gets a triple kill is pretty impressive too. She takes herself, a newborn baby and Ilithyia out all at the same time — by giving Ilythia a C-Section and then throwing herself and the newborn baby off a cliff!
Hell, the fact that the only thing that can kill Lucretia is herself is rather awesome. People have tried, namely Ilythia and Crixus, but it takes Lucretia actively trying to kill herself to do it. And she does it in style.
War of the Damned
Marcus Crassus gets one in the very first episode, cementing what a truly Magnificent Bastard he is by manipulating Spartacus into killing the Roman generals in charge of the legions, thereby putting him in command of the entire force. He's also being taught how to fight like a gladiator, learning Spartacus' own fighting style, something his own son thinks is 'beneath him.'
Marcus knowing that he needs to know his opponent tells the audience how capable a villain he is. His "final test" with his gladiator trainer is a thing of awesome, as well.
Spartacus and his army successfully taking over Sinuessa en Valle.
In the second episode, Crassus' wife is informed that an important guest of his is growing impatient at having to wait for her husband. She tells the guards to send him on his way. The man does not take the news lightly, and completely curb-stomps the guards with his bare hands. Crassus returns, sees the scene and shouts just one word: "CAESAR!" Awesome because it completely subverts the usual image we have of Caesar: he's unpolished, younger, rasher. Not someone who you'd think will one day rule an empire...
Caesar again in "Decimation". He's been sent by Crassus to infiltrate the captured city and find out what he can about Spartacus' strategies. He immediately zeroes in on one of the less-than-content members of the rebel army, convinces him that he's on their side. And, with just a few words, incites an already restless mob into killing every single Roman held captive and driving a deep wedge between Spartacus and Crixus, who had allowed the massacre to happen. Crixus and Naevia are going to go off on their own, Spartacus' forces are divided...and Caesar smirks.
And then in "Blood Brothers", Caesar betrays Nemetes and kills him, despite his pleas of mercy. He manages to set the city's gate on fire, but gets cornered by Agron, Saxa, and Donar. Then a battering ram smashes through the gate. Caesar's final line (and trademark smirk) says it all: "Now, would be time to run!"
Ceasar actually holds his own against all three of them, notably fighting off Agron and Donar at the same time. Not one then the other and back, but both trying to kill him at once. Ceasar keeps maneuvering to keep them from flanking him, gets them in the way of each other's attacks, and generally fights exactly like you're supposed to when facing two opponents at once.
Spartacus revealing that the fracturing between him and Crixus was all part of a huge Xanatos Gambit to divide Crassus' forces. It even fooled most viewers, who thought that the ending of 'Decimation' would be the part where Crixus would go off on his own with part of the rebel army.
Crassus, however, sees through Spartacus' ploy and sends the entirety of his army to Sinuessa at the end of "Blood Brothers."
Saxa beating down two gladiators to save a bunch of Romans. "I give cause, bitch."
In "Spoils of War", Gannicus dealing Caesar a Curb-Stomp Battle when he easily holds off him and the Roman contingent with him to buy time for Laeta and Sibyl to escape. Caesar, who had barely defeated one of the captured Gladiators before despite the gladiator's wound, challenged him and...ended up on the receiving end of Gannicus using a scissor cut right on his stomach. If not for the rest of Caesar's men, Gannicus would have killed him.
Laeta stabbing Heracleo through the neck with the hot iron he branded her with.
Crassus again showing how savvy he is. Throughout the episode, he's been surprisingly nonchalant about the Rebel's escape. Then the final scene reveals that he had a massive wall built on the Rebel's escape route. There is nowhere to run.
"Mors Idecepta" gives us another from Crassus, who anticipates Spartacus sneaking into his camp with a few soldiers and makes sure that they find poor Donar, dead and crucified with the episode's title ("Death is undeceivable") carved on his chest. The rebels just make it out alive, Naevia heavily injured. And then Spartacus tops that moment when he realizes that there are barricades over the wall to give the illusion of a massive army behind them. Cue Curb-Stomp Battle of the few hundred Roman soldiers and Spartacus and his army finally free from the ridge.
The way he even breaches the trench dividing the wall and the barricade is awesome, if morbid: The frozen bodies of their comrades who died during the violent snowstorm are piled high until the survivors can walk on top of them.
When Crassus and his troops investigate the breach, they are met with spears and are forced to retreat. Whilst they are escaping Crixus boasts "The mighty Crassus shows f*cking arse as he FLEES from us!", followed by triumphant jeers from the Rebels.
Kore should be mentioned here. Tiberius warned her that if she told his father about how he raped her, no-one would ever believe her and she would be punished. She manages to get freaking Caesar to sneak her out of Sinuessa so she can tell Crassus the news (keeping in mind that for all Crassus loves her, she's a slave and is actively plotting against her master's eldest son)...only for him to choose the worst words possible (which amount to "The bond between my son and I are absolute and nothing can ever break that"). Faced with the choice between risking Crassus' wrath if she tells him the truth, or keeping quiet and staying in Sinuessa to serve under Tiberius and be raped indefinitely, Kore takes a third option... and joins Spartacus' rebellion, killing the sentry placed outside Crassus' tent.
Across the board all over The Dead and the Dying:
Spartacus sending his men to pretend to be envoys of Pompey to lure Crassus into a trap. This isn't why this moment is awesome. It's awesome because Caesar sees through their disguises and convinces Crassus to send Tiberius instead, thereby getting his revenge for his rape in the previous episode.
The games that Spartacus organizes to remember all their fallen friends. Each named character gets a chance to shine in the mock-arena.
During these fights Spartacus fights two Romans and when one of them manages to cut him across the cheek he discards his swords and fights both the armed men with nothing but his bare hands.
Gannicus trumps Spartacus by challenging three Romans and beating them without so much as a scratch.
Kore coming across Tiberius and his men, chained up. He begs her to rescue him, even using Crassus' feelings for her to manipulate her, only for Kore to reject him utterly.
In fact, all three people that Tiberius has caused harm to get their chance at revenge. Caesar sends him to his doom, Naevia utterly dominates him in the arena and Kore, his first victim, is the one who kills him just as he's about to be taken by Caesar. And as she realizes her mistake, Kore immediately offers to take Tiberius' place so the trade can continue, not knowing how Crassus will react when he sees her - his runaway slave - again.
And finally: the end of the episode. As Naevia lights up Crixus' funeral pyre, everyone begins to chant the names of their fallen friends, one by one. Not just from this season, but from the very beginning: Varro, Barca, Mira, Oenemaus...and it culminates in everyone shouting "FOR CRIXUS!" as the pyre reaches its peak.
The series finale, while a full-on Tearjerker in its own right, still had its awesome moments.
Lugo still swinging his giant mace while burning alive from a well-aimed catapult, managing to kill one more Roman soldier before screaming 'Fuck your mothers!' in German as his face melts away.
Gannicus seeing his Elysium - an arena full of screaming fans and Oenomaus smiling at him - and letting out one last full-throated roar before he dies from crucifixion.
Spartacus vs. Crassus. Crassus is injured and is carried out of the battle for a Medicus. Then Spartacus comes running up the hill, and kills every single one of Crassus' guards before their duel. And the best part? If those guards hadn't intervened and impaled Spartacus with spears, he would have won. Even Crassus is awed, and wishes that Spartacus had been born a Roman, as he's probably the only person in his life that he respects.
Two things make it better than that; firstly, while Crassus has a minor wound, Spartacus is obviously ragged by the time he gets to the Imperator, meaning that the fight was only as even as it was because Spartacus himself was considerably less than 100 percent. Secondly, is Spartacus' reply to Crassus' wish that he had been born Roman; "It is a blessed fate that it is not so."
When Spartacus blocks the maneuver that killed Hilarus.
The montage at the end of the episode, complete with awesome Latin chanting and an image of the fallen. It ends with Liam McIntyre as usual....and then immediately cuts to Andy Whitfield screaming 'I AM SPARTACUS!'
Even the way the characters were grouped together counts. The villains of the various seasons were all together, Spartacus' lovers were all together, and the leaders of the rebellion were together. And Onemaus, Melita, and Gannicus are together.