Frankly, all the treks are similiar.. seasons 1-3 meh to blah, seasons 4-6, pretty good, season 7 depends.
Even Enterprise , who had a much shorter run, knew it was in danger of cancelation and after season two , kicked it up a few notches.
My list if it "only" had 5 would be, and Im regulating to "space" shows vs "scifi" shows.
1. BSG (updated), it might have been the best character driven, well produced show Ive ever seen. Even if it was more a delve into the darkness of human phycosis then it was about humanities ablity to overcome. However, you want to view the show, it set standards of how a science fiction series can be made. Ive cried twice in the history of my watching science fiction, Spocks death scence and the end of BSG, if a series can illicit such reactions, you know its doing something right.
2. Farscape, by the end of season 4, I can see alot of repeated themes in this show, frankly , for how much they might have wanted to extend this show, it probably should have found its conclusions in that years series. To me this show was at its peak from 1.5 to the death of Zhaan or maybe Crais, I dont think some of thier characters were ever fully challenged or flushed out as they should be. It also stradled alot of character development, Chriton the protagonist, seems to be a unflinchingly heroic charater, to a selfish man, willing to sacrafice everything to get his way. Also, it remains annoying that EVERY episode the darn ship, or someone was just about to die. However, go compare the custume work from this show vs startrek tng universe, Henson company did a bang up job here. The graphics also are pretty nice , even in todays world. It tried to capture a feel and I think it did that. I dont want to bad mouth the series too much, its on my list as #2 for several reasons. For one thing, how can you not love a scifi show that would give shout outs to other scifi shows! Yoda, Worf, Klingons, trek even Buffy =). They also couldve done a few episodes centered around the secondary characters, growing them and thier stories, this might have extended the show in a much more non linear way they were trying to do.
3. Star Trek Voyager, I really think this was some of the most worthy moments of trek history from Seven of Nine to the end. It was truly a treat to see Sevens growth, the interactions of the crew around having a borg on board, plus you have to admit the show was ten times better when they changed antagonist to the Borg. The show really started off slow, but built, just like TSG and DS9 in my opionion, however overall I would rate Voyager as slightly ahead of both of those in its overal content. I just cant get over how terrible the start of ds9 was, I know some of you will disagree, but there is a reason they made as many changes as they did.
4. Firefly, I dont care if it was only on one year. It burnt bright it the short time it was on. I am still pissed off to this day what Fox did to this show. How SHORT sighted can one network be? This was never properly promoted, never given a good slot, never show in ORDER even!!!! Im sorry, but to this day, I just cant believe how ripped off we were. Imagine if this show lasted 4 years even!
5. Red Dwarf, Yes its campy, but go watch them, then come back to me and say it wasnt sci fi. Its cold outside, with no kind of atmosphere... I think we all have a little lister and a little rimmer inside all of us =).
Not on my list,
Dr Who.. its more scifi, then space based scifi, so it would be in my top 5 of my whole complete list for sure, as would star trek TOS, but if your judging a show by its ships, the Tardis, is more a vechicle to a destination and never a vechicle you lived in , for the show purpose anyways.
B5, story arc aside, I just dont think this is a top 5 show, either in the space side , or full list side. It never drew me in like other shows, I struggled to get through them at times. There is a reason you share story writing responsibilites at times, fresh perspectives, do help!
DS9, early DS9 was just the worst of trek IMO only, it grows, it grows dramatically , but I cant get past those first couple of years, the worst of DS9 is far more horrible then the worst of TSG or Voyager.
TSG, This grows as well, from borg/picard onward, the show is much better. There is a episode, where they focus on lesser officers for a whole show, its really quite unique in the trekverse, I wish TSG highlight moments were earlier and more often, but the best of TSG still holds well.
There is a HUGE list of other shows that I consider to be the best of scifi, but none of them are overally spaced themed, like xfiles, outerlimits (new one were the best non arced scifi on telivision really).
Not only is Earth 2 not really a space themed scifi, it wasnt even well made, I regretted less loosing that show, then loosing surface. Heroes was terrible after year 1 and I quickly abandoned it.
All the stargates are ok, but I dont think many would be considered as pure space scifi.