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".
The best part of that 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.
Episode 10 "Wrath of the Gods": Glaber getting the mercenaries to turn on Ashur by simply offering 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!"
Episode 1 "Fugitivus": Mira describes Agron as "An angry boy who barely piss without splashing everyone about him."
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."
Episode 2 "Wolves At The Gate": Julius Caesar orders a slave girl to circumcise him. He seems to get off on it. Tiberius walks in on them and is disturbed by this.
Turns out that it wasn't what it looked like- she was cutting into his leg to help with his cover for when he infiltrated the rebels. Still, his apparent pleasure was real enough, and Tiberius' reaction is still hilarious.
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 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."
Also from Episode 6, this time by Caesar. "Oh, and Tiberius? Oysters."
Episode 9 "The Dead and Dying": Kore leading Tiberius on that she might rescue him just to see him get his hopes up and dashed when she tells him she'll enjoy seeing him die.
And again from episode 9 — the episode 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."