Every time someone swears, especially that time Batiatus screamed, "At long last, cock has been removed from ass!"
Every time someone sings "My Cock Rages On".
The sheer number of ways the male characters can use references to their dicks as a metaphor for ANYTHING.
Blood and Sand
Episode 2 "Sacramentum Gladitorum": Barca kills one of the recruits in an offhanded fashion.
Batiatus: Not every venture ends in climax.
Lucretia: A fact well known to many women.
Episode 3 "Legends": Spartacus taunts Gnaeus until he angrily charges. Spartacus sidesteps and Gnaeus runs head-first into a post and knocks himself out.
After Spartacus dismisses the symbol of Missio (mercy), and Varro attempts to smooth things over with Doctore, both are sent "to the pit" for punishment. Gilligan Cut to them in basically a garbage well, soaked up to their waists in filth, and someone dumping some more refuse in. Varro glares at Spartacus, Spartacus looks back a little sheepishly and says "Perhaps I spoke out of turn."
The exchange between Gnaeus and Crixus. Gaeus briefly laughs when he discovers they are to face of and warns "I will fuck your corpse". Crixus jovially replies "With what cock?"; both of them resume laughing.
Episode 4 "The Thing in the Pit": Spartacus starts talking to Varro and his hallucination of Sura at the same time, making the conversation very confusing for Varro.
Episode 5: "Shadow Games": Gnaeus collapses from heat exhaustion. When Pietros brings water, Oenomaus tells him to spare it for more worthy men. Que Pietros walking away from a fainted Gnaeus.
Episode 7: "Great and Unfortunate Things": The slow motion image of Gnaeus falling over the ridge. A still image is even featured during the final end credits of the show.
Episode 8 "Mark of the Brotherhood": When the new recruits are ordered to undress, everyone's reaction to Segovax amounts to, "Holy shit, that guy's dick is huge." Varro says he's betting on "the one with the horse-cock". Even more Hilarious in Hindsight after Vengeance aired: Agron takes particular interest.
Batiatus: Patronage comes with unfortunate attachments.
Spartacus: (rattles chains) A fact I am well aware of.
Gods of the Arena
Episode 1 "Past Transgressions": Gannicus gets drunk and almost walks off the cliff.
Episode 1 "Fugitivus": Mira describes Agron as "An angry boy who barely pisses without splashing everyone about him."
In the supposedly abandoned ludus, Ilithyia makes a shocking discovery and screams in terror!
Marcus: (rushing into the hall) What has happened?
Glaber: (calmly heading in the direction of the scream) In matters concerning my wife the gods themselves fear to speculate.
Agron becomes rather snarky in this season, such as when Spartacus says he'll train his new recruits in the same manner he was trained.
Agron: And look how well that turned out.
Episode 7 "Sacramentum": Ashur's failure at intimidation:
Ashur: I would advise caution. These men are the hands of Praetor Gaius Claudius Glaber.
Gannicus: And what are you? His cock or his ass?
Episode 8 "Balance": Mira walking in on Agron and Nasir trying to have sex while on guard duty. Not that she seems to mind. The best part of the scene is that after being caught, Agron and Nasir spend about 5 minutes going "Um..we were..uh" and generally looking like schoolchildren caught making out under the bleachers.
Episode 9 "Monsters": Oenemaus knocks Agron to the ground. Nasir laughs at him.
During the battle at the end of the episode.
Roman officer: "Praetor! The rebels are in retreat!" * Spartacus and Gannicus immediately appear out of nowhere and slaughter everybody (except the Praetor, who they take captive)*
Episode 10 "Wrath of the Gods": Glaber getting the mercenaries to turn on Ashur simply by offering them more money. Ashur pretty much goes, "Son of a-"
When the Romans try to interrogate one of the captured Germans, the man yells insults in German like, "Suck my dick!"
War of the Damned
Episode 1 "Enemies of Rome": Gannicus kicks a man out of his own tent so he and Saxa can have a four-way with two hot chicks. Later, when Spartacus orders him to give his battle report, Gannicus answers, "We fought, we won."
Sanus: (Saxa brings in the two women) Your woman brings gifts!
Saxa: (knocks him down) Gifts are not for you.
Episode 2 "Wolves At The Gate": Julius Caesar orders a slave girl to cut him in the thigh. He seems to get off on it. Tiberius walks in on them and is disturbed by this.
Episode 3 "Men of Honor": A drunk rebel runs around naked and boasts, "My cock is magic!" Disgusted, Crixus replies, "See it vanish from sight."
Episode 4 "Decimation": Agron has taken on the role of Spartacus's bodyguard, shadowing him everywhere. At one point, they enter a bar and are approached by topless women. The look on Agron's face as he pushes one away is priceless.
Episode 6 "Spoils of War": Combining heartfelt drama with humour.
Gannicus: I will cause distraction and gain needed time. Spartacus: By what means? Gannicus: I have no fucking idea.
Caesar, who is naked in his room with two practically-naked female bed slaves. Tiberius had previously asked if there was anything special he wanted served at the celebration of his victorious retaking of Sinuessa en Valle, and, after some hostile repartee, is about to leave in disgust. "Oh, and Tiberius? Oysters."
In Episode 8 "Separate Paths", Spartacus and Agron heed Kore's call for help and rush to her tent to find a woman in labor undressed and moaning on the floor. Spartacus instantly stares at the ceiling, and both men proceed to look profoundly uncomfortable until Kore calls for Agron's knife to cut the cord.
Episode 9 "The Dead and Dying": Kore leading Tiberius on, claiming that she might rescue him just to see him get his hopes up, and dashing them when she tells him she'll enjoy seeing him die.
Episode 9 opens with the standard slaughter of Roman troops, beginning with one man taking an arrow to the face. During the melee, repeated shots show Saxa, in the background, running up to attack — except by the time she arrives the slaughter is done.
Saxa: Time next, you carry bow. Too far from fucking blood.