Well the Fourth Doctor (Tom Baker) was certainly the runaway favorite in the poll I put up, and I've finally gotten around to him. Hopefully these 12 different variations should be enough for everyone...
Baker is arguably the archetype for DW Doctors, and he's certainly the one I remember seeing on PBS as a kid - Even though I never really understood it and was far more interested in Superman and Batman. I loved the theme music though!
In fact, when the show came back with Eccleston, I said "Hey, what is this?! Doctor Who is supposed to have a hat and a long scarf!" Obviously I still knew very little about Doctor Who then, but now I understand why Tom Baker is such a beloved Doctor.
And as a bonus, check out some cool DW artwork people have been inspired to do -
First, 'John Smith's' 'The Implausible Astronaut'
Then we have 'Doctor Who on the Simpsons' by CPD-91
And also, something really cool from 'Michael'...