Star Trek! It had it all. Completely original and unlike anything that ever came before it, I would say that Star Trek set the stage for other sci fi TV series to follow. It is the example that all others have attempted to duplicate.
While sci fi series have been popular for decades and many shows have dedicated followings (such as Star Trek and The Twilight Zone) the best series is Battlestar Galactica. The quality of the show far surpases any other. It is complex, exciting, and also has quite a following.