He is probably best known for The Prisoner, a bizarre, surrealistic take on the 60s spy genre that he helped create, and even wrote episodes for under the name Paddy Fitz. In the show he played a spy who resigned for unknown reasons, someone, it's never identified who, kidnapped him and placed him on The Island, a strange, creepily pleasant prison for people with secrets.
His captors take away his name, which is never revealed, call him Number 6, and do everything in their power to break his mind and spirit to steal his secret. He becomes the Island's rebel declaring at the beginning of each episode: "I am not a number! I am a free man!"
As a kid it was my first taste of Kafkaesque existentialism, and it was an experiment that was ahead of its time in many ways.
He was also notorious for turning down the role of James Bond, twice. His reason for the refusal was because he considered Bond an "amoral thug" and thought the character's constant seductions would be disrespectful to his wife.
He was more than just an actor, or even a star, he was an artistic ground-breaker who helped took television past the edge in ways that are only now being appreciated.
Rest in peace Mr. McGoohan, you were never a number, and always will be a free man.