I like:
Star Trek in all forms, TNG and DS9 are my top two favorites, and TOS was always there my whole life, so I have a sentimental attachment to that one. VOYAGER and ENTERPRISE are my least favorites, but both have some good stuff in them
Babylon 5. If Star Trek hadn't been created, this would be the greatest thing that ever happened to Sci-Fi. An excellent series from beginning to end.
The Twilight Zone. The original is the best, but the 1980's version had some good episodes too. The 2003 series was a waste.
Buck Rogers (1979). Not serious Sci-Fi, but fun and enjoyable. The first season was the best. The second just seemed like a Star Trek rip-off, but even that had some good stuff.
Battlestar Galactica. The 1978 version is better than Ron Moore's version, but even that does have some good stuff in it. Even if Kara Thrace is painful to watch.
Farscape. An excellent series. Like Buck Rogers, it's both fun and enjoyable.
Firefly. A classic in the making that got killed before it's time. Too bad.
Earth 2. Enjoyable, but nothing special. One season was enough, but the lack of a conclusion was disapointing.
SeaQuest. Started out great, but turned into crap after the first year and never recoverd.
Sliders. See SeaQuest.
The four Irwin Allen series from the 1960's. Lost in Space, Voyage to the Bottom of the Sea, Land of the Giants, and Time Tunnel. Time Tunnel was the best, but the others were enjoyable.
Voyagers. Fun travel show.
Quantum Leap. As fair as I'm concerned, this was the best attempt (so far) at a time travel series.
V. The original was a mixed bag. I've only seen the first two episodes of the new one, but those were okay.
Caprica. I've seen every episode so far and can't seem to make up my mind about it.
Taken 2002 miniseries. I enjoyed this so much, I watch my DVD set at least one a year.