For me, the Best 'single' Doctor of the revamped Doctor Who has to be Tennant, he really picked up the ball running from Eccleston and made the character his own, by the end of his tenure I was seeing him as 'the' Doctor.
Smith however I think has been part of the best ensemble, I think Matts Doctor, Amy and Rory (who's grown on me immensely after what felt like his intrusion in the first of Matt's seasons) are better as a TEAM then the random assortments Tennant was stuck with. As such, whilst Tennant is perhaps the best on his own, I think Matt Smith wins out for me!