Artorias I imagine. I still need dlc but I loved 1-3 1-4 boss fights from demon's. I'm a big fan of duel-like fights. Currently I'd give it to S&O.
Least favourite is BoC, but it hardly counts as a boss. Actual boss is iron giant. I don't have problems with him but it's an awful fight. The whole grab = instant death thing based on your luck is just lame. Sif's pretty bad too just because of how repetitive the fight is.