First Doctor

Played by William Hartnell in the original run, and then by other actors after his death, the First Doctor lasted for three years before regenerating.

Personally, I have never been a fan of the First Doctor. I find him far too grumpy to be likeable, with little charm of the later incarnations, and his way of laughing to himself regularly every time he has an idea is immensely irritating. However, I am prepared to cut him some slack as the show was just getting started and Hartnell obviously did a good enough job to ensure that it wasn't terminated after a single season.