I'm going to have to agree with the OP Malboros from FFVIII were awfullllllll I don't I ever stuck around to actually kill one without Odin. Ultimecia wasn't HARD per se but just annoying and way too many phases.

Snipers...Call of Duty!

I'm playing Mario 64 DS again... so I have to say those flying fire spitters shy guys! Fuck, they're hard to hit!

Gary Oak.

Malboro was so easy.  Well,in a straight fight, yeah, but I you can use Quistis' Degenerator(Blue Magic) for easy kill but without it, a strong juntion-stats defense against bad breath and you can give Malboro a good fight.

I think Alma/Alma II from Ninja Gaiden Black. She is very strong and really fast and very hard, even in normal mode. I don't know how attacks against her work but sometimes when you attack her weak point, she just deflect my attack, it wasn't hard to hit her weak point, I think it was something random that the attack hits successfully, and worst yet, you have to be moving around all the time to avoid her attacks and those crab enemies too.

this X100

I can't believe I'm the first one to say Ozma from FF IX. That damn fight was the bane of my existence.

The final battle in Orzammar in Dragon Age was a beast as well. Really, the whole three-boss sequence (starting with BroodMother) was a real pain. Way worse than the High Dragon.

Hmm... Ozma, Orzammar... odd coincidence. lol.

Ozma is amongst the easiest FF superbosses, I think that's why no one mentioned it yet.

I mean, you can even beat it at Lvl. 1

yeah, the Malboros were a really pain in tha ass.

The Medusa Heads from Castlevania.