1963-1966
The very first Doctor to start the series. A very no-nonsense old man, very knowledgeable, and not afraid to express it. We are introduced to the series that he travels with his granddaughter in the TARDIS through time and space. He collapses on the floor of the TARDIS in his last episode after fighting Cybermen and regenerates.
1966-1969
The second Doctor, the first to follow another actor in the position. He was much more jovial than his predecessor, though under this lighthearted guise he always had a intellectual and hardhitting interior that presented itself when necessary. In his final episode, he contacts the Time Lords (his people) for help, and he gets in trouble for continual interference with other species. He is forced into regeneration by the Time Lords, condemned to Earth and the secrets of the TARDIS are taken away by the Time Lords, stopping him from travelling.
1970-1974
The Third Doctor, he was very much a "James Bond" type Doctor, very martial arts, gadgets, and fast cars. Many of his episodes were on Earth with the UNIT organization due to being prevented from using the TARDIS. Eventually he's given back permission to use the TARDIS. He regenerates after heavy exposure to radiation on the spider planet.
1974-1981
The Fourth Doctor, and certainly the most famous. He had the longest stretch on the show. Armed with sardonic wit, a toothy grin, and a loyal robotic dog K-9, this Doctor won many people's hearts on his adventures and brought the show higher ratings than before. He regenerated after falling from a high radio telescope tower.
1981-1984
The Fifth Doctor, a fan of the game of cricket and a calm and cool adventurer under pressure. Carrying a celery stalk around to detect when danger was imminent, this Doctor brought a lot of depth to the roll. He regenerated after being poisoned with Spectrox and gave the only antidote to his companion.
1984-1986
The Sixth Doctor, a much more sarcastic and edgier Doctor than the previous, almost to a fault. Many of his episodes concerned with the Trial of a TimeLord. He regenerates after the TARDIS is forced to crashland into a planet by the Rani.
1986-1989, 1996
The Seventh Doctor, a very clownish yet extremely mysterious Doctor. At one moment he would play the spoons and whistle to birds, the next he would whisper about secrets of gods and the universe's origin. He was the last Doctor on the original series to end in 1989, and appeared briefly for his regeneration scene in the 1996 movie where he shot by a gang in San Francisco.
1996
The Eighth Doctor, only appearing in the 1996 Fox movie, though had many books and webisodes written about his adventures. He was the first Doctor to be romantic with one of his companions, which caused a lot of uproar. His regeneration is never shown, but it is assumed he suffers fatal wounds as a result of the last great Time War.
2005
The Ninth Doctor, the first Doctor to appear in the new series, returning to television after a 16 year absence. A hardened Time Lord after what we can imagine a brutal Time War, he is a little less merciful on foes than in the past. He is the second Doctor to enter into a romance with his companion. He regenerates after absorbing the Time Vortex from his companion, Rose Tyler.
2006-?
The Tenth and most current Doctor. A little more lighthearted than his previous incarnation, yet still as hardened with his "1 chance" rule. we look forward to many great, new episodes with our newest Time Lord!