My favorite is Patrick Troughton by a long shot. Second favorite is Peter Davison, but Matt Smith is rapidly climbing up the charts and he's challenging.
It's difficult for me to come up with a "least favorite" Doctor. Colin Baker's stories were terrible but I love his audio work. I've been surprised when going back through episodes that I find it difficult to rewatch a lot of the Tennant episodes for some reason. I enjoyed him a lot as the Doctor but on rewatching it he's becoming very grating and melodramatic. I don't think I'd call him the worst, but it's interesting.
That's interesting, I caught the last fine minutes or so of The Satan Pit the other night and God Tennant was annoying (in fairness though he was at his most annoying in series 2, he did calm down) and I have to say this is something that's become more apparent since he left whenever I watch an episode. He's so in your face and hyper and trying so very, very hard, and at times its painful to watch. Of course I acclimatise to him--and the hyper loud, hyper shouty RTD Who-- very quickly and enjoy it, but it's still oh so obvious that Smith is far more natural, relaxed and comfortable in the Doctor's skin than Tennant often was.