Best Boss? Hmm... I would have to say Diablo's clone (Worldstone quest) in Diablo II: Lord Of Destruction in the v1.10 on Hell Hardcore mode. I dont know about popularity, but he is graphically perfect, great moves, talks shit when u come to kill him, and is probably the most difficult Boss I have faced in any video game.
Wow! Do i sound like a gamer nerd or what?
P.S. Anybody play Diablo II in here?