Well... since I've only ever seen christopher Eccleston and David Tennant, i'm not sure. I do love Christopher, but again, David would have to top my (oh so informed) books.
David Tennant! Everytime I watch an episode with the 10th doctor I walk away happy, even in the most depressing episodes!! I can't put my finger on what Tennant brings to the table that I love so much, but I would almost say the 10th doctor is my all-time favorite character, regardless of the show. I'm sure the writing has a good deal to do with this as well. I think it's a prime example of harmony between actor and writer.
David Tennant's probably my fave but i do enjoy the wit and style of Matt Smith. saw every episode of the first season of Matt Smith as the Doctor. Gotta wait till around Christmas for the new season.