This is a toughy, far too many good episodes to choose from.
1) I'll cheat a bit by including the 4 episodes realeased under Sozin's comet as 1.
2) The siege of the Northern tribe part 2.
3) & 4) Boiling rock part 1 and 2
5) Sokka's master
6) The cross roads of Destiny
It's weird that most of the above episodes are from season 3 just shows how good that season was.