This is going out on a limb, but I'm going to say that Stargate is my pick for the best TV SF show.
-Dunks and hides-
Please note I didn't say it was the best SF ever produced, just that it was the best SF TV show. I look to TV for entertainment and I think Stargate had the most consistant entertainment value. The production values were good, without ever having SPFX over power other areas of the show. The the cannon plotline is convoluted to the extreme and often downright silly if you stopped to think about it, but the writing was clever, and the writers were obviously confortable and familiar with the characters (Unlike say, Enterprise where the writers seemed they had never heard of the Trek Universe). The show has the right balance of humor and seriousness, but never takes it's self too seriously. There was great on screen cast chemistry, (something that just happens and can't be forced), and good casting of small character parts (I think second only to X-files in the genre).
All this makes great entertainment, it's just fun to watch the show, and it does what I want a TV show to do; take me away from the 9-5 grind of the real world.