Being the Avatar freak that I am, I would probably say the Zuko/Azula fight during the season three finale.
Psycho Azula verses vengeful and "good guy" Zuko is epic enough; add in the amazingly mournful song that started off after the first blast, that scene where Zuko is nearly killed by lightening by saving Katara, and Azula's mental breakdown in the end......couldn't ask for a better fight scene, really. It was breathtaking - ASTOUNDING!
