The Dark Queen from Ico is pretty unstoppable.....given you dont care to find your sword. If there were no blocks....yeah, she'd be the hardest boss ever. lol
Aside from that, Wherever the sword lands is all the same pattern, or same location each time. So she's not that bad. Just cool.
As far as bosses go...I can mention the high level monsters (might as well treat them like bosses) from DnD, but that isn't a Video Game, so ah well.
Soooo I'd nominate
. Just hope that you don't have A BIT OF BAD LUCK
but even then, for some people, hard can be easy. Not to mention I feel a bit depressed that Sephi from KH2 is easy to alot and I die from him a fair bit o' times before I defeat him. Same case for KH1 Sephi lol