Dark Souls offers some of the best and obviously hardest boss fights in any game, ever. But the hardest are between the Four Kings and Seath the Scaleless. Especially on each subsequent playthrough in which they increase in strength by an absurd amount.
Four Kings is hard because it needs to be quick, but it is very difficult to deal high damage to them and stay healed. Every now and then another King will spawn, so depending on how slow you are, you could end up being attacked by 3 or 4 bosses at once. This becomes flat-out impossible to defend against, with some of their attacks (grabs and magic) being unblockable.
Seath is hard because he can curse you, and without the right armour set, he can do this very easily. He can raise crystals from the floor that fill up your curse meter when they touch you, and sometimes he literally fills the room with them leaving you no escape. If this (small) curse meter fills up, you instantly die and respawn with half your HP, making it nightmarish to lift the curse if you don't have the items you need to do so.
Yet the masochist in me is still playing, currently on NG+3 and outside the Four Kings' area. Wish me luck.