After having time to think about it, I still feel Ornstein and Smough is the toughest boss I have experienced. I'm talking 3-4 hours unable to defeat them, having to go back and readjust my armor/weapon/style before finally defeating him.
Makes me want to play that game again oddly enough.
"You lack the requisite spine and testicular fortitude to study under me"
"You do not know the first note of the music that moves me."