Tom is a national treasure (a much overused phrase, but genuinely true in this case). I think the reason his Doctor was so successful and remains so popluar is that Tom is a genuine eccentric and just played the part as himself so the eccentricity came across as genuinely off-the-wall and unique, whereas the other actors who played the Doctor were not naturally eccentric so when they try to play it like that it seems forced (David Tennant I'm looking at you!). I've had the pleasure of meeting him briefly twice so far, the first when I was only 5 and he was still playing the role on TV - he turned up at a wolworths in full costume! And second in '97 when he signed my copy of his autobiography and offered me a jelly baby. What a moment!
My favourite Dr Who stories of Tom's are: City of Death, The Ark in Space, The Seeds of Doom, The Robots of Death and The Pirate Planet.