I picked Hillary though she wasn't technically a nominee. The only reason she lost to a guy who voted present in his short career with her huge liberal gravitas and advantage was because a few big Hollywood types decided it was time to elect a black guy.
Of those who got the nomination, I would say Kerry. He was a complete disaster of a nominee. His Swift Boating was one of the most insidious and effective guerrilla campaigns I have seen in my lifetime, not that it wasn't warranted.