Matt Smith. He's got great range as an actor, a lot of energy that lubricates even the dumbest scripts, and a face that sometimes looks leading-man-ish, but just as often doesn't quite look human. I just wish more stories had been deserving of him.
After that, it gets tough. Frankly, I love them all for one reason or another. I suppose my next favorite is Tom Baker -- "my" Doctor. But on different days, Troughton, McGann, Eccleston, Tennant, and even McCoy might take that spot.