Whenever Kawamura's split personality shows. In episode 20 Momo deliberately activates it in this situation:
Ishida: "Excuse me, how is your right arm?"
Kawamura: "Ah, it's fine. It wasn't your fault, so don't worry about it."
Momoshiro: "Here Taka-san." *hands him his racket*
Kawamura: *in BURNING mode* "HEY YOU! YOU HIT A PRETTY ROUGH BALL AT ME, BRAT!"
From the same episode, Ryoma's reaction to the surprise party is to promptly close the door on his teammates.
The day after playing bloodshot Kirihara, Ryoma admits to himself that he was a pretty scary opponent. Unfortunately for him, all his teammates, the freshman trio, and even Coach Ryuzaki have red eyes for various reasons. This freaks out Ryoma to the point of fainting. TWICE.
Just watch Kaidoh becoming everyone's Butt Monkey in "Kaoru's misfortune".
Any time Kaidoh and Momishiro have to play doubles, actually. The Comedy Tennis was just the height of their antics.
Tezuka Zone. Dinosaurs. 'Nuff said.
The revelation about Atobe's wig.
The entire Yakiniku episode, but particularly Shishido's "Heroic Sacrifice" (complete with Ohtori's "SHISHIDO-SAAAAAAN!!! ;-;") and when Tezuka accidentally yanks down Inui's pants.
Any time Inui loses his pants, in fact.
All of the chibi episodes, and the "[insertnameofsport] no oujisama" ones. Oishi tends to get especially funny in them (like in the bowling episode, and when he's the mom in the chibi family episodes...). Also, Tezuka and bonsais... And most of the mini-dramas in the radio program.
Atobe and Ryoma's maniacal laughter right before their match
Let's not forget episode 99, "The Cursed Racket." We have Inui's ghost story, the attempted racket exorcisms, poor Kaidoh getting abused again... not to mention everyone's hilarious terrified faces throughout.
In the anime rendition of the Yamabuki arc, Sumire calls Nanjiro by phone. As they speak, she mentions Banji... and accidentally presses Nanjirou's Berserk Button by doing so. Hilarity Ensues BIG TIME.