Love Doctor Who, and I've been watching it since Tom Baker, the Fourth Doctor. For sentimental reasons (I used to curl up with my mother and sister on the couch, and watch the rerun episodes on PBS) he will always be my favorite - and as far as companions go? I've got a soft spot for Sarah Jane Smith and K-9 too.
I do enjoy all the newer incarnations as well, though I'm partial to Tennant simply for his quirky and wholly original take, one that really seemed to breathe life back into the public interest in the series. I never did like the companion Martha, because [generally useless], and Rose got irritating after a while as well, with that overblown, shark-jumping romantic neediness.