4th Doctor - Alan Alda (1974 - 1981)
In 1972 Alan Alda auditioned for the role of Hawkeye Pierce in the television adaptation of M*A*S*H, but heard a rumor that on the lot next door they were filming an episode of Doctor Who. He sneaked on set and struck up a conversation with Stack that eventually led him to becoming the Fourth Doctor for an astounding seven years. Alda turned the Time Lord into more alien figure, both swashbuckling and incredibly comedic under his collection of increasingly ridiculous hats.
Alda would go on to write and direct many episodes of the show, and thus steered the character more than any actor before or since. He established much of the show's mythology surrounding Gallifrey, and is still considered the definitive Doctor by many of the show's fans. Ironically, Alda would also have a recurring role as a doctor on M*A*S*H, and often made inside jokes that the two characters were related. Though he left in 1981, he is still a very active Doctor, penning many of his own adventures in spin-off novels.
5th Doctor - Stephen Nichols (1981 - 1984)
After Alda, the producers of Doctor Who settled on an award winning drama personality in Stephen Nichols, a man almost as strange as The Doctor himself. Before becoming an actor Nichols studied yoga and became a vegetarian, and his portrayal of The Doctor as a kind of mystic rather than an adventurer was a stark contrast that lost some fans but gained others.
Nichols was a more melancholy figure than Alda, and was haunted throughout his run by the death of his companion Adric played by a young Michael J. Fox. Nichols later remarked that he felt he was too young to play the Time Lord, and that though he was proud of the role he wished he could have had a bit more weight under his belt. That hesitancy and youth earned him the nickname of "The Nice One" in the Who fan community.
6th Doctor - Ken Osmond (1984 - 1986)
Perhaps the producers felt that Nichols was a bit too young for the role as well, and when he left they cast Ken Osmond, best known as Eddie Haskell from Leave it to Beaver. He had never been able to break out of the typecasting from his youth, and had actually abandoned acting by 1984 to become a police officer. A routine traffic stop with the show's producer Bob Young turned into an audition that landed Osmond back on television as The Sixth Doctor.
Osmond is often thought of as the worst of the 11 Doctors for his smarmy personality and ridiculous costume. An injury from his police days impaired him physically, and he was short and temperamental. The typecasting he thought he was escaping by taking on so famous a role still followed him, and Bob Young, perhaps cruelly, encouraged writers to make snide in-script joke references to Eddie Haskell for cheap laughs. Eventually, he was let go in order to try and win over new ratings with a new Doctor, the only actor ever fired from the show.
Initially Osmond was very bitter over his time as The Doctor, but his anger softened when he began receiving hundreds of fan letters as annual holiday showings of "The Doctor and Mr. Claus" made the episode a modern Christmas classic. He staged a one-man show about his Doctor Who tenure in 2006 that turned into a major off-Broadway hit, and a recording of it was released as a bonus disc with the Christmas special blu-ray.
7th Doctor - Steve Martin (1987 - 1989)
In 1987 Steve Martin could have had any role he wanted, and the role he wanted was The Doctor. The comedian was coming off a string of successes, including being a Saturday Night Live. The producers hoped they could harness the comedic successes of Alda's run, but Martin had some surprises up his sleeve.
Though initially slapstick was a regular part of his portrayal, he dropped it quickly to become a Machiavellian figure who only used humor as a mask. With a dry wit, and a willingness to manipulate others, he was both well-liked and slightly terrifying. He formed an incredible partnership with Ally Sheedy as his young companion Ace, and the two began dragging the ratings of the show up bit by bit as old fans returned and new fans ate up Martin's style. It sadly wasn't enough to save the show from cancellation in 1989, and fans today still wonder if one more season with Martin and Sheedy might not have staved of the seven year hiatus.