I - Probably Tiamat. After the easy bosses so far, that one was a kick to the face.
II - Astaroth. I almost died on my last playthrough because of him!
III - Garuda. We all know why.
IV - I have bad memories of this game. To me, I kinda hated all the game bosses for being too annoying.
V - I guess X-Death. No one else really compared to that horrendous fight where he can get you STONED!
X - Final Aeon. Yowza! What a jerk!