I have to say I love it when Toph calls Aang Twickle-toes. I crack up!

Or the time in "the blind bandit" when the gang sits down in front and someones like "I wonder why nobody is sitting in front." and then a rock comes and smashes right next to them.

I forget now which episode... I think Burning Rock Pt 1... when Zuko pretends to be Iroh (and says something to the effect that even when he is pretending to be Iroh, he still can't understand what his uncle is saying)

i love toph's little lines that she says throughout the show. especially when she is making jokes about her being blind. one of my favorites though is
"I AM MELON LORD!!" XD

How about "sparky sparky boom man!"

It doesn't matter how many times i watch that line i laugh every time.