Sokka giving a Firebender a Groin Attack from behind during the Earthbender fight. Also one of the funniest moments of the movie.
The scene with the Blue Spirit was pretty epic. The action was almost shot-by-shot from the cartoon.
The part where Zuko ninjas into the Northern Water Tribe; also was pretty much the same as the cartoon.
Aang bending a giant wave of water at the climax. Yeah, it wasn't Koizilla, but it was still pretty darn cool. Also, the fact that doing so meant he was finally willing to deal with his grief over losing his people and accept his destiny as the Avatar.
The fight between Aang and Zuko at the North Pole is probably the best fight scene in the whole film, aside from the Blue Spirit fight. Also, the fact that during the fight when Aang hides behind Zuko, he's actually using a martial arts move that Aang used on Zuko the first time they fought in the series.