Hmm...My faves for KH2 would probably be, in this order:
1) 1,000 Heartless Battle (pure gold)
2) Axel the 2nd time (dual-wielding goodness)
3) Sora/Riku duel at Land of the Dragons (I like Riku a lot)
4) Roxas' battle with the giant Nobody at the Station of Serenity (gotta love those reaction commands)
5) Saix (just an all-around good challenge, I thought)
...I'm a little disappointed that this list is hardly original, and I could probably come up with a more accurate one if I actually went through my guide and picked out my favorite fights from a big list.
P.S.: Xaldin = runner-up for being able to play as Mickey (if you suck).