For me it has to be Tom Baker. My brother was into the show when I was a kid and he's the first I'd ever seen in the role. Always loved his "all over the place" style, which has stuck with the character ever since.
Tennant for now is my 2nd. I'm still catching up with the old series so that may change sometime in future. He reminds me more of the 5th Doctor than anyone, especially after "Time Crash". "You were MY Doctor."
Even for all the haters out there, I like Colin Baker. Yes he could be a bit of an ass at times, but that made for some interesting stories.