Colin Baker, by a mile.
I'm gonna go against many people and say the McCoy was a really good Doctor. He's definitely in the top half. Yes there were some crappy stories (especially in his first season), but there were some good ones as well (I really liked Silver Nemesis, Battlefield, Remembrance, Fenric, Survival).
Colin Baker, though... ugh.
From the old series:
Hartnell: grumpy old man
Troughton: bit of a clown, flighty, panics
Pertwee: action man, "snazzy" dresser, kindly gentleman
Tom Baker (my favourite): joker, tried to be mysterious
Davison: weak, always a step behind
Colin Baker: arrogant arsehole, worst Doctor evuh
McCoy: brough some sense of fun back, had some good and _awful_ scripts
McGann... I try to forget that film.
Eccleston: a good way to reboot. Chipper, not too serious, dynamic
Tennant: started off a little too wild and short-attention-span but settled down. In the end I think I liked him more than Eccleston.
Smith: started far too young and was far too weak; too many echoes of Tennant. But grew into the role.
Capaldi: grumpy old arsehole. Not liking him, so far.
Now I'm hoping this upcoming series will be better. I think there's potential, but so far he's the worst of the new Doctors and possibly the second worst Doctor thus far.