Sokka's fantastic Moment Killer scene when he walks in on Aang and Katara's passionate kiss, and constantly referring to them as "oogie" afterwards, much to Katara's annoyance. Makes it better, the moment that he killed was originally the last scene of the series, and an extremely emotional one at that. Hilarious.
At the end of Part 1, Toph demonstrates to Katara exactly what "oogies" are.
Katara: (Hugs Aang) I just hate seeing you get like that, sweetie. Sokka: Um, Aang? Katara? You're giving me the oogies over here! Katara: Aargh! I am so sick of hearing about your stupid "oogies," Sokka! I don't even know what that means! Toph: I can explain. Give me your hand. Katara: (Gives Toph her hand) Toph: Gaaah... (Licks Katara's hand◊) Katara: ! Scream Discretion Shot: OOGIE!
Best part is how Toph apparently has Gene Simmons' tongue. Slides it riiight up Katara's ring finger.
Toph fires her first blind joke of the interquel;
Toph: Hey, wanna know what fireworks are like for me? Close your eyes. (Sokka does so) Toph: (leans close to Sokka's ear) BOOOOM! Sokka: AAAH!
The three students at Toph's metalbending academy have some funny lines between them. "The city's probably doomed! We're probably doomed!" "I hate doom."
"The Dark One"'s name is actually, "Moo-Chee-Goo-Chee-La-Poo-Chee The Third." Overly long name being funny? Oh, yes, but think about it: he's the third one of his family named that and absolutely no one thought that this was a bad idea.
The crowning moment is probably the otherwise heartwarming moment when they reveal how Toph having expectations for them helped them unlock their metalbending.
Ho-Tun: Nobody's ever expected me to be anything other than a... a... a wimp. Penga: ...a spoiled brat. The Dark One: ...a tall, dark, mysterioushunk. Sokka: Really? =__= The Dark One: Why can't people see I'm more than just a pretty face?! I have the soul of a poet!
After Toph stops the Earth Kingdom rioters:
Sokka: Good thing you listened to her, folks! I was about to bust out Mr. Boomerang! Whack-a-pow!
Random Guy: Ooooh! "Mr. Boomerang"! Soooo scary!
(Sokka hits guy in the head with "Mr. Boomerang")
Plenty of moments when Sokka assists Toph's Metalbending class.
Sokka: You have to not only see the metal. But hear the metal. Taste the metal. And BE the metal!
Iroh: First I cook balls of tapioca until they're soft and tender. Then I put them in tea, where they sit like little pearl-sized snacks at the bottom of each cup! Add a little milk and — ta da! — A revolution in tea is born!
Zuko: *PTAH* What is that trying to sneak into my mouth?!
Aang: I've never had tea that's quite so... Chewy.