Ornstein and Smough ... the music and the place and all makes it an awesome battle also Artorias of the Abyss. Hated the 4 kings
I also hate the 4 Kings. I've fought them probably 50 times, and they're just a terribly unbalanced boss battle. I'd say 4 Kings is the 3rd worst boss.
I feel it's pretty unanimous that the Bed of Chaos is most people's least favorite. Second worst, for me, goes to Gwyndolin, with his 3 predictable, easily dodged attacks.