It's not actually a line but here it goes...
It's from "The Swamp" and I just took this transcript from AvatarSpirit:"Katara: What was that!
Sokka: Nothing. Just swamp gas. (The gas begins to swirl about them) Look, there’s nothing supernatural going on here.
All: (As the gas reaches there noses) Ugh!
(Their disgust in interrupted by a terrifying scream. The stop in mid retch and grab each other with bugged out eyes. The camera zooms backward to reveal a white bird in the foreground with a huge mouth. This is the source of the scream. It screams once more and flies off. Cut back to the terrified kids still gripping each other)-> I owe this bird the best laugh of my life Sokka: I think we should build a fire…"
And from this episode you also have the wonderful scene from Sokka:"(Cut back to an overhead shot of Sokka and Katara walking through the swamp. Cut to a shot of a crocodile like monster sits in the water some distance away, then cut back to Katara who points at Sokka.)
Katara: Sokka! You’ve got an elbow leech!
Sokka: (freaking out) Where! Where! -> I had this as my text message ringtone for weeks (After he stops moving, it is clear the leech is hanging from his elbow. Katara crosses her arms and looks coolly at her brother.)
Katara: Where do you think
Sokka: (angrily ripping off the leech and throwing it near Katara) Why do things keep attaching to me!"
I mean... there are so many it's hard to choose. I've just remembered this two 'cause I was watching the show in the afternoon...