I expected Tennant and Mad Tom to be the favorites, and the actual poll seems to be bearing this out. I suspect it's due to their comparatively long tenures in the role (Tennant has just matched Hartnell's, Troughton's, Davison's, C. Baker's, and McCoy's durations and is under contract to at least match Pertwee's) along with the fact that for many viewers their first Doctor was one of them. Tom Baker was the first seen by many Americans of my generation, and I suspect that many who have recently seen Doctor Who for the first time came into it seeing Tennant.
Plus, they're both really, really good in the role.
Thoughts?