I'm astonished Babylon 5 hasn't been mentioned here, since it must have been a risk at the outset. The writers knew where they wanted to take the show and were given the leeway to go and do it. In a way it was only spoilt by the fact that having got to the end of the story in a satisfactory manner, the TV company then commissioned another season, which must be just about the only time this has happened in sci fi television history.