Sephiroth and Kurt Zisa for sure, I never beat Sephiroth (he was quite easy for me in KHII though). I'm not sure if I beat Kurt Zisa or not, I can't remember, if I did, it was only on Normal.
Ice Titan is nowhere near as hard, I remember him being relatively easy and fun.
--
~Halo Give into the Night.
http://i10.photobucket.com/albums/a125/ShadowHalo/haloshadow1.gif