Star Trek (the orginal series)
I'm serious. Came out during the cold war, and gave hope for the future. First interracial kiss on American tv.
By placing the setting in the future and in space, the writers could bring controversial subjects to light in a way no other show of it's time could.
Without Star Trek, there's likely no Star Wars, no Battlestar Galactica, no X-Files, no Fringe, no Firefly.