Christopher insists Whittaker owned the role in her first episode, but not only do I disagree with that, rather strongly too, I don't think any Doctor owned the part as fast and as completely as Matt Smith did in this brilliant, perfect post-regeneration episode. Several come close (Troughton, Pertwee, Tom Baker, Eccleston), but none of them are quite this combination of being a fantastic story in its own right, a brilliant performance by a noobie Doctor and a phenomenal statement for the new state of affairs for this regenerated, at this point, show (though Troughton and Pertwee are pretty damn close).
This really a wonderful, amazingly great episode, that I always loved and always will. Long live the Doctor, indeed!