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.
“Anna, my strong, terrifying Anna.” -Cas
"You lack the requisite spine and testicular fortitude to study under me" -Elodin