4. Tom Baker
The legendary Fourth Doctor is the longest tenured Doctor to date, occupying the role from 1974 to 1981. Even after his regeneration in the final episode of 1981, neither Peter Davison, Colin Baker (no relation), nor Sylvester McCoy could really escape the shadow of Tom Baker. Bakers reign as the traveling Time Lord saw some of the greatest stories to date, including two of my personal favorites, The Ark in Space and City of Death. At the end of the Logopolis serial, the Fourth Doctor takes a nasty fall from an observation tower during a confrontation with main antagonist The Master. As he lay dying, his companions call out to him, past and present, in a similar fashion as the later regeneration of Peter Davison, symbolizing the people who care about him and need him to live. The Doctor then merged with a mysterious incarnation of himself known as The Watcher, thus beginning the regeneration he had struggled to achieve and becomes Peter Davison. The Tom Baker era was over, the shadow of which would be cast on the show for years to come.