It is no secret among Whovians (as we like to call ourselves) that the longevity of everybodys favorite Time Lord is largely due to his regenerative powers. For those who are fairly new to the franchise, in the event that a Time Lord, the Doctors alien race, is fatally wounded for any reason, he can regenerate up to twelve times (this fact may very well be disputed in the near future). The downside of this power is that the regeneration energy regenerates mind as well as body, thus each regeneration means a completely new Doctor that shares memories of past incarnations. This allows new actors to put their own spin to the Doctors character. Each regeneration is particularly heart wrenching in its own rite, depending on ones opinion of each Doctor, but these five are easily the most memorable.
5. Patrick Troughton
Okay, just because I list this regeneration as memorable, I dont mean its actually good. Patrick Troughton, one of my personal favorite Doctors (and for you Potter fans out there, the grandfather of Dudley Dursley actor Harry Melling), took over the blue box in 1966 due to the failing health of the original Billy Hartnell. It was under the helm of Troughtons childish, recorder playing Doctor where we first met the stoic Brigadier Lethbridge-Stewart and Corporal Benton, and he would later return periodically as the result of time paradoxes throughout the shows 26 year run until his death in 1987. Troughtons reign as the madman with a box came to an end in 1969, when the Time Lords put the Doctor on trial for meddling in the affairs of other species, notably mankind. These same charges were brought to retrial in the unwatchable Trial of a Time Lord serial in 1986. The Time Lords determine that while the Doctors actions were well intentioned, he could not be allowed to interfere any further. The council decreed that the secret of the TARDIS would be forcibly taken from him and his next incarnation, played by Jon Pertwee, would be exiled to Earth for the majority of his run as the Doctor. His subsequent regeneration then reminds of you of the worst LSD trip imaginable multiplied by about ten.