Seriously, this is a very hard decision for me to make. Each of the Doctors are unique and lovable, and I fell in love with all of them. But, if forced to decide, I would say that the 10th Doctor is the best. He's optimistic and funny, and you just fall in love with him at the first scene that he shows up in, right after the 9th Doctor regenerates.
I believe that the 10th Doctor rules overall. He's just unbeatable. David Tennant is a god and none of the other Doctors compare. HOWEVER I strongly believe that your first Doctor will always be your favorite. I fell in love with 10 and I don't feel like the others compare. If my first had been any of the others I would probably feel this way towards them too though :) <3