Depends upon the genre. But a few of them Monty Python's Flying Circus: genius comedy that redefined what everyone on the planet considered funny.
Blake's 7: A show that defined real character & story in a time of 2D heroes & contrived situations (ie Trek)
Play School/Sesame St: These shows took an educational approach to entertaining children & made it work so well they continue to be the standard that others are measured by.