Inferno, from Soul Calibur 2, in ultra hard level, (happens at the end after defeating 9 enemies in the same level), but in an arcade machine, definitively the hardest I have done.
Demon Soul and Ninja Gaiden Bosses passed my mind when seeing the title ..
both are the hardest game i ever played for now ... especially about the bosses
5.Jecht - FF10 <-- not really that hard but his background music is badass
4.Sephiroth - Kingdom Hearts (Hard Mode) <-- 1combo kill
3.Yiazmat - FF12 <-- 50million HP, status attacks and AOEs
2.Freya - Star Ocean 3 (Universe Mode) <-- HARD, nuff said
1.HitoShura - Digital Devil Saga <-- whoever included this guy in DDS is a f*ckin sicko! XD
Seph from KH.(still haven't defeated him)
Yiazmat FF12.
Took me 2 days to kill and I was already level 99 with the strongest weapons in the game.Damn Square Enix.
He is near impossible if you dont have the Dragon killer axe and dragon killer card.
Actually replaying the game and completing all of the trade quests just for them.
Well the hardest boss I've ever fought is probably Panthera Cantus (Mr. Hanekoma) from the "Another Day" chapter in The World Ends With You. The best I could do was on medium; I don't know how anyone could beat it on Ultimate and one level one for that matter...
Fighting Vercingetorix in FFXIII underleveled was a total misake that got my ass handed to me ten times over. I am now currently leveling up and upgrading for rematch number 11.