When Ichigo finds out about Karin's crush on Hitsugaya
Ichigo: He dies. Hollow: *Falls over laughing* Zangetsu: (trying not to laugh) Perhaps a bit excessive, Ichigo. It is unlikely he reciprocated, or even knew about it. He was probably just bored. Ichigo: He'd fucking better not have! He's at least ten times her age! Can I maim him? Zangetsu: It has not happened yet, and now probably never will. Ichigo: Fine. Still gonna kick his ass Hollow: Yes! Dragon-boy's going down!
Made even funnier by the fact that the entire Gotei 13 are watching via a surveillance device. Hitsugaya's Oh Crap reaction is priceless.
Pretty much all of the bit where the Gotei 13 are watching Ichigo's interactions with Muramasa counts, as they provide a rather humorous running commentary of the events. It helps that they're treating it like a movie night.
Ichigo: I can probably give Ishida and Inoue a challenge in the class rankings now, since I'll have done most of the material before. It's gonna be pretty funny seeing Ishida's face when he realizes that. Bastard'll start studying twice as hard just to beat me, though, and he's pretty smart.
Ishida: You're damn right I will! That's not fair!