For me personally, I prefer Smith to Tennant, simply because he's more quirky and harkens back more to the Doctors of old... Like Baker. (Tom Baker that is, not the other one). Think most people would agree though that both were better then Eccleston. While he was a good Doctor to reboot the series with, he didn't have the charisma to carry it like Tennant and Smith.
I believe most people prefer Tennant, simply as he was the Doctor when the show started to really take off in popularity again.
Anyway.... Baker, the 4th Doctor was certainly the best in my opinion. Quirky, charismatic, yet perfectly capable of doing dark if he had to.