First, I'll have to divide between "classic" (1977 and before) and "modern" (after 1977); with Star Wars, the whole approach to sci-fi television changed
"classic" top 5:
Twilight Zone - pretty much a given, it's influenced most good (and a lot of bad) sci-fi since
Outer Limits - not as good, but still memorable
ST:TOS - again, this set the standard for space sci-fi to follow, for good and bad
The Prisoner - maybe a tough call as sci-fi, but advanced electronics, mind control, rockets and a giant animated ball combine to put it in this category. Plus, it really messed me up as a child.
Batman - yes, it's campy and set back comics years, but it's still a lot of fun
honorable semi-genre mentions:
Wild Wild West - steampunk before there was the word
Voyage to the Bottom of the Sea - submarine adventure was the space adventure of its day. "Seaquest" certainly didn't beat it
"classic" bottom 5:
Lost in Space - I thought it was dumb as a kid, and still don't like it
Land of the Lost - liked it a little more, but has aged very badly
Dr Who - I know this is heresy, but while there are many good episodes, they are outweighed by the bad
Electra Woman and DynaGirl - just plain bad
Time Tunnel - best of the worst, but has been done better by later shows
Disappointing mention:
Six Million Dollar Man - great intro and first years, but had a long slide downhill
"Modern" Top 5 (no particular order):
SG-1/Atlantis: Not high art, but conistent sci-fi that does a good balance between episodic stories and plot arcs. Probably the most family-friendly scifi currently out, with a strong non-cheesecake female character and not much in the way of "mature" themes (my 13 yo enjoys this a lot more than BSG)
DS9: Not pure Trek, but the best Trek Universe show. Got too mystical at the end, but still several strong seasons.
Firefly: Had great potential, and some great episodes.
Farscape: certainly a "love it or hate it" show. I happen to love it, as a show that was brave enough to both try bizarre ideas and to not take itself too seriously.
BSG (new version): here on probationary status; hopefully, the second season will be as good
Honorable mention: B5 - had 3 of the best-written seasons of sci-fi, but the acting and mediocre seasons 1 and 5 keep it just off the last
"Modern" bottom 5:
Final Season Award (shared) - "Battlestar 80", "Buck Rogers", "War of the Worlds" - all fair to decent shows that were ruined by a change in the core concept
NBC Superhero Award (shared) - Manimal and Powers of Matthew Star - both too short to get their own slot. And don't forget "Misfits of Science"!
V (the series) - miniseries was great, but the series simply ran out of steam and budget. when a 10 yo can spot recycled, mis-matched footage, you've got a problem.
ABC Superhero Award (shared) - Automan and "The Phoenix" - early 80's was a bad time for TV scifi
ST Voyager - best of the worst; could have been great, and had some great episodes, but degenerated into "The 7-of-9 and Doctor Show" near the end, and had a simply awful finale
Disappointing mention:
Crusade - Ted Turner and JMS do not make a good combination. Could have been a good show (Starblazers/Yamato in B5), but clashes ruined it
But they were a blast to work on, and I can console myself that there is an obvious quality gap between Herc and Xena, and, say, Sinbad or The New Adventures of Robin Hood.)
