To be honest, it was the movie that made me go back and watch the entire series. I wouldn't say any one episode hooked me, as much as the premise. I loved the series, though I was disappointed with the end (only felt that should have provided more resolution after, instead of the short 5 minutes we did see, maybe one more episode after just to tie up all of the loose ends nicely I think.

The last 5-6 episodes of the 2nd season. I also feel that the final resolution was a little bit to short, though there is a comic series regarding the events after the final episode (unfortunatelly i have not read them yet)

I get hooked when i saw the first episode lol but the one that really hooked me is Avatar Roku: Winter Solstice, Part 2

The Southern Air Temple episode was when I got hooked, but The Storm episode made me a fan for life.

Probably the episode where Toph first appeared in person (not in the swarm episode).