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!
Gin: He's right. That's a funny face.
While trying to convince Muramasa to take a new wielder, Ichigo mentions friends of his who might be willing to take a zanpakutou...and some who wouldn't.
Ichigo: Ishida would laugh at me if I asked him to become a shinigami.
Ishida: Yes, I would.
When Ichigo shows Muramasa his memory of Tatsuki punching his head through a window, Kenpachi calls dibs.
Many moments involving Mayuri, especially when Ichigo shows the events of the Fade to Black movie.
The image showed Ichigo standing with Hisagi and a group of shinigami from the Ninth as giant worm-like creatures rising from the advancing ooze attacked them.
Yamamoto decided that an audit of the current experiments at the RDI was in order.
And later in that scene:
Mayuri: There are a dozen things they could have used. Be glad that the one they chose was relatively nonfatal!"
Yamamoto decided to make that audit a priority.
Ichigo's reaction to his hollow form. Made even more hilarious by how Zangetsu is trying not to laugh.
Ichigo: Oh, no. No. I ain't fighting like this. I look like a lizard.
And the Call-Back later, when Tosen has gone into his resureccion form, which looks vaguely like a fly.
'Lizard' was fine. He could have had it so much worse.