i'm havent seen all that many episodes of the old ones but i really loved David Tennant (10), but Christopher Eccleston (9) is good too. David Tennant's goofy eyes and eyebrows were just so expressive and fitting of the character. The second doctor was Patrick Troughton.
David Tennant has left the show and is being replaced by Matt Smith (11) and his companion will be Amy Pond (played by Karen Gillan). The new season starts in Jan 2010. For a cheesy British show the concept are very original.