My list:
1)Kurt Zisa (took me forever, that cheap spin move took me forever to learn how to avoid)
2)Ice Titan (those damned icicles whenever you used aroga...i eventually found out how to block them) (by the way I never used a strategy guide, except on DMC3, I had to buy it to figure out the game, and I still can't get past Agni and Rudra)
3)Phantom (I had to figure out the spells and about being able to stop the clock...I figured it out pretty fast though I ain't that stupid...)
4)Ansem (I just beat his *** into the ground, but he hit me a couple times...didnt do much though, amazing what having a level 100 party with the best weapons [save the king/queen and ultima keyblade] can do)
5)Sephiroth (him...he made me use a few potions from that sin harvest thing whenever I didn't stop it, but other than that it was all dodge slash away, dodge, slash him into the ground, dodge, repeat, etc. etc.)