ahh man i think i have to dispute this one. probabally the hardest boss i have ever faced (and thats me not everyone else) would probabally be ultimecia from ff8, until i figured out that you have to use lionheart, and the aura spell...normaly i would agree with euclid, but when i went to fight ultemicia the first time, i had all three guy characters leveled up to 100 (i didnt really use the chicks) and in the end it got down to just zell & irvine. i kept using limit breaks on her bitch ass, because she kept doing the attack that gets you down to 1 hp, and after like 30 min of this, she still wasnt dead, and she killed me off. i eventually finished the job.
oh also i think we must make mention of the last two bosses in parasite eve (eve and her baby). damn all those transformations drove me nuts in that game. just when you think everythings over, they transform again! i remember when i got to the point where i had to get off the ship or whatever so that i wouldnt get killed by the boiler room explosion, my dad wanted to talk to me about something, so i let the game sit in the middle of some dialogue, thinking it wouldnt progress, cause i wasnt pressing anything. about 10 min later, after the conversation, i came back to find out i had died!! after all that fighting of the baby freak, I WAS GONNA HAVE TO DO IT AGAIN!! (luckily i had my action replay to help me out, hey at that point i knew i coulda beat it. its not like there is anymore combat when your leading the thing around)