I'm looking at the actors who played the doctor, it is sort of crazy how most of them only last 3-4 years except for the fourth doctor who had a longer then normal run. I don't think there is any TV show that has had so many different lead actor changes:
1. First Doctor, played by William Hartnell (1963–1966)
2. Second Doctor, played by Patrick Troughton (1966–1969)
3. Third Doctor, played by Jon Pertwee (1970–1974)
4. Fourth Doctor, played by Tom Baker (1974–1981)
5. Fifth Doctor, played by Peter Davison (1981–1984)
6. Sixth Doctor, played by Colin Baker (1984–1986)
7. Seventh Doctor, played by Sylvester McCoy (1987–1989, 1996)
8. Eighth Doctor, played by Paul McGann (1996)
9. Ninth Doctor, played by Christopher Eccleston (2005)
10. Tenth Doctor, played by David Tennant (2005–present)