Without out a doubt Star Trek.
It set the tone (or reflected, depending) of what the future could and should be, which had the downwind affect of other shows either reacting to it or taking the ball and running with it with their own takes.
And then there are the sequels, the movies, the novels; all of it casting several generations to look for a better future (well, except for the current imaginings). It has a breadth and reach that cannot be underestimated.
If we are talking most influential for today, Babylon 5. It is the first long arc tv show (and one of the best), which nearly every series owes a debt to in some way.
