I think that The Doctor was at his best when played by David Tennant so far. A lot of purists and fans of the original older series would disagree with this, but to be frank, they're like two different shows. I think that we could almost separate them in this respect. Tennant is a very accomplished actor in other fields as well; if you've seen a clip of his performance in Hamlet, you may see what I mean.
Anyway, Matt Smith isn't a bad Doctor. I think that he nailed the essence of his own inner Doctor archetype (which is all The Doctor at his core really is; aside from that, virtually nothing is consistent with this character, especially over the course of the old series). However, Smith's episodes are not as well done as would suit his good rendition of the character.
I think that the main failing is actually with Amy and Rory and the incessant focus on them as characters. The last few episodes have all been about how important their connection is. Yes, they're married, it's lovely, we get it. Can we have some Doctor Who action now?
In my view, the David Tennant seasons really rammed home the theme of The Doctor's survival, his attempts and failures regarding mercy and justice and these in conflict with his warrior side, and left us hungering for more of that. Smith's episodes are written in a way that is much more whimsical and humorous, but perhaps lacking in meaning. I just think that more could be done with the central character. He's more than just a reflection of his companions.