Hardest bosses for me were most of the optional ones, especially Sephy from KH2.

If we're talking storyline bosses:

KH1:
- Maleficent, took quite a few tries to win it, I think. It was a long time ago

KH2:
- Xaldin, that took some serious concentration to win
- Demyx, most overall deaths in a boss fight due to those damn timed water clones lousy fighter my ass...