Definetely Tennant for me. Very early in his run, after I had gotten over the "huh, this is weird" feeling due to a new Doctor I realised I thought he's a brilliant actor. This seems to be a minority opinion now in this forum but I always found his intense, expressive acting style fitting and great to watch. And I usually prefer understated acting. Anyway, his work on the show made me check out anything he was in I could get a hold of. I even watched all the Harry Potter movies because he has a few scenes in the fourth one. And he's just great in Hamlet.
I also like to see Eccleston in other things. Matt Smith hasn't really wowed me, yet. And it's probably not that easy to get to watch other stuff Hartnell and Troughton were in, though in the case of Troughton it would probably be very interesting because he looks so different from his Doctor persona. That about covers all the Doctors I know.