Jon Pertwee (1919-1996) from '70-'74.
Regenerated due to the fact that he was poisoned by radiation on the planet Metabelis III.
'Planet of the Spiders' is absolutely wonderful. I have watched it and would definitely recommend it to any Whovian.
Conveyed by Patrick Troughton (1920-1987) from '66-'69.
Regenerated as a punishment established by the Time Lords and exiled to Earth.
Episode title: 'The War Games'. I watched it and the episodes were epic!!!!!! <3