Favorite and least favorite boss fights by game (though KH 1, 2, and 3 are the only ones I remember well):
KH1
Favorite: Darkside, Guard Armor, Chernabog, Riku-Ansem, Ansem (second fight), Kurt Zisa, Xemnas (even though I've never beaten him)
Least favorite: Trickmaster, Clayton/Stealth Sneak...