I would have to go with Star Trek. Although I much prefer The Next Generation over the original series; the original was great. If you look at all the cancelled series, none have become as influential as Star Trek.
Lets hope the upcomming Star Wars series is good!
While I wouldn't call either of them the best Sci-Fi, there are two shows I want to mention. The Sentinal and 7 Days. I loved both shows when they aired on UPN after ST:Voyager. Neither lasted longer than a few seasons, though.
Just FYI, if you used to like either show but haven't seen them in a while:
Sci-Fi channel plays reruns of The Sentinal, but I'm not sure when.
SpikeTV plays 7 Days weekdays at 9:00am central time.