Another vote for Babylon 5. I loved Sliders until the second season, when they stopped doing cool alternate-history scripts and started doing pale imitations of zombie movie scripts, etc.
Lexx.
Can’t believe there’s not more love for DS9. Was it just that it seemed so good at the time in comparison to all the other Star Treks, or is it that it seems worse now because it’s associated with all the other Star Treks?
I would have thought the consensus would be
B5
DS9
Farscape
Futurama (it’s a comedy, sure, but why should that disqualify it?)
Gap
SG-1
Huge light-years sized Gap
Everything else
** Buffy/Angel not eligible
Here’s another vote for DS9, and another vote for Buffy and Angel being fantasy, not scifi. (the closest either got to sci fi were the computer demon that Willow almost hooked up with, Adam, and the Buffy-bot (and her predecessor-bot). Borderline, but still, I say fantasy. Same for Highlander and Forever Knight.
EDIT: Oh, and Ted. I forgot Ted.
I think that voting for Firefly as best sci-fi show of the 90s is similar to the way that Jesus Christ got so many votes for person of the century for the 1900’s, despite dying nearly 2000 years ago.
I thought it was shit at the time, and I still think it’s shit - a rip-off of the B5 concept* with typical Trek windowdressing that simultaneously breaks down the myth of a better future of the earlier treks and replaces it with…a shittier myth of secret police and wormhole aliens. Baah!
(*yes, I know they came out at the same time, but JMS has indicated Paramount had seen the B5 concept as early as '89)
Of those I’ve seen (and I’ve not watched more than an episode or two apiece of Babylon 5 or Red Dwarf)…
Futurama is tops. Yes, it was parody. Yes, it was disjointed episodes with little to no overarching storyline, just a few wisps like “Fry pines for the 20th century” or “Leela wants to know from where she came.” But it’s both a brilliant send-up of science fiction tropes, and just a hugely entertaining sci-fi show in its own right. There’s a reason it’s still a cornerstone of Adult Swim’s lineup after four years off the air (and effectively butchered airing for a year or two before that. Thanks FOX!), and why they’re going back to create additional material.
And I still hold a torch for the oft-forgotten Dark Skies, which is a rarity: Compelling television sci-fi alt-history.
I couldn’t even get through the first episode of Babylon 5. The acting was so incredibly bad. But I’ve heard so many good things that I’ll give it another try.
My vote is for The X-Files. Well-written, intriguing, and very much not your standard sci-fi or horror or mystery fare. Plus, it was consistently quality television through all 7 of its seasons. Futurama is a close second.
You might want to start with Season 2 then. There is enough recap in the first few episodes to get you up to partial speed. Better yet, watch “Passing Through Gethsemane” from Season 3, then start Season 2.
I’m bringing my wife into the Babylon 5 'verse this way. So far she is enjoying the heck out of it.
Yeah, a lot of the first season of Babylon 5 is pretty painful. It gets MUCH better, enough that it more than makes up for a rocky beginning. The last six episodes or so of the first season really pick up the pace story-wise, and then it’s generally smooth sailing from there on.
Farscape fan here!
My vote is for The X-Files as well, for the same reasons. By the way, didn’t it last nine seasons, or are you intentionally dropping the last two? 
(Poor Agent Doggett - better than most characters on TV, but not quite Mulder…)
And once they finally drop Michael “Solid Oak” O’Hare, the acting improves immeasurably as well. There are a couple of third-season eps in which Sinclair returns briefly, and shock of just how HORRIBLE he is after two seasons without him is like bellyflopping into the Arctic Ocean in January.
And yet, those episodes are so cool, I didn’t really mind him. Some of their cast came from stage and soap opera backgrounds and reeeeaaaally didn’t know how to dial it back for TV. A lot of people got better, and some people just left. And then there was the redhead who played Lyta Alexander, whose idea of acting was to squint a lot. God, I hated her.
But the show is awesome! Really! I swear!
Well, seeing as how Patricia Tallman was primarily a stuntperson before B5, I cut her a little slack. Besides, she redeemed herself by getting nekkid with a bunch of telepaths and hitting G’Kar with the single best line of the whole series!
“Oh, and you mentioned wondering what my pleasure threshold is. I just recently found out. … I don’t have one. Have a very, very nice day, G’Kar.”
I know, and I’m sure she’s a very nice person. But I don’t go around performing neurosurgery, paralyzing people, and then using the excuse that it’s okay because my old job was as a writer. It’s still important to be good at what you do. That was a very good line, and I always wish it had gone to someone else.
There are a lot of Sci-fi TV series that I liked the first season or two, but they end end jumping the shark… (Like when Farscape or Lexx end up at Earth, or when the SeaQuest left Earth. Wince)
The most consistant, for me was Babylon 5 and the Star Trek series.
Those that had potential unrealised:
Space: Above and Beyond, Crusade.