Astoria tricks Cerebus into giving her some water in Church and State II. Even when tied up, fatigued, and in pain, she can still do what she does best.
Cerebus snapping out of his Heroic BSOD and killing the two Cirinists at the end of Melmoth; made all the more rewarding by the fact that these were the ones who imprisoned Jaka and Rick in the previous phonebooks.
Cerebus killing Thrunk.
Po basically scaring Cirin into ordering her guards to leave and hearing him out in Reads.
Cerebus killing Cirin and all of her guards without even breaking a sweat.
Yeah, it's just a possible outcome that Po mentions to Cirin, but Sim at least has the decency to show it and it's pretty cool (one of the FEW cool things about Reads).
The giant aardvark stature functioning as a Humongous Mecha in an alternate future mentioned in Minds.
Baskin killing a Cirinist assassin and saving Lord Julius in Flight.
Dave Sim completing Issue 300, as he promised back in the day. Even his ex-friend Jeff Smith briefly put aside the animosity to congratulate him and Gerhard.
Cerebus finally standing up to Astoria in the conclusion of High Society, sending the normally calm and collected woman into a Villainous Breakdown.
Cerebus' Cirinist Rampage in the beginning of Flight. He takes on several scores of Cirinists Warriors and easily cuts them down.
The fight between Cerebus and Cirin in Reads, particularly the climax of the scene where Cerebus, after being kicked and thrown around for nearly the whole fight, leaps on Cirin's shoulder and proceeds to smash his fist into her face several times. The scene is made even more awesome when we learn what Cerebus's thought process was before that moment during Minds.
Cerebus: Cerebus hasn't got any strength left, Tarim..Cerebus has lost too much blood..hacked just about to pieces..Cerebus is going to...jump..grab..Cirin's sword arm..Help Cerebus, Tarim..Give..Cerebus..The Strength..
Oscar gives Pud a verbal smackdown when he calls him a "Somdomite" in Jaka's Story.
The radio adaptation of Cerebus destroying the Pigt idol, which is notably more dramatic than the comic version (the fact that it is accompanied by Mars Bringer of War certainly helps).