Isshin the Saint from Sekiro. I mean really? four phases? in a game so difficult that it trips up dark souls players? thats insane, I still have not beaten him. Genichiro beat me a lot to, and I considered that the fight of my life.

took me no small amount of tries to beat the Abyss Watchers in Dark Souls 3.

from a much easier game: for some reason, in Paper Mario I think I've only ever beaten the master toad fighter at the dojo once, and never beaten the anti guys, or kent C. koopa. those three optional bosses and the amayzee dayzees are straight up the hardest enemies to fight in Paper Mario.