If got a lot of TV series and am a sci-fi nut so this is my territory totally...
B5 - Brilliant, shame they $%&#ed them about and made them finish it in S4 only to have to start over again in S5 leaving S5 a bit patchy. Good stuff none the less.
Firefly was magnificent and it was a travesty what Fox did to it. Serenity went some way to making it right but it just wasn't enough. Shame, one of the best sci-fi's ever killed due to studio bumbling incompetance.
Lost - S1 was awesome, but it became clear about halfway through S2 that they hadn't actually written any more plot and were just making it up as they went along to milk it for as long as possible (X Files anyone...). I gave in on that there and then.
Same goes for all the Lost-a-likes (Flash Forward et-al).
Really found myself enjoying Stargate Universe. Kinda a matured SG franchise. At least they got a chance to give it an acceptable ending.
Watched 3 seasons of BSG and loved it. Got the lot on a shelf and still haven't got around to finishing it... Must do that.
If you can deal with the muppets (grew up with the muppet show so no probs there) and the fact it never really took itself seriously, Farscape was bloody brilliant. They couldn't have killed that one at a worse place (the end of S4 was a bit errm shocking to say the least). That was a show with a very definite 5 year arc and at least again, fan pressure got some sort of closure. Though cramming an entire season worth of plot closure into a 4 hour mini series was a little uncomfortable pace wise (and it lost a lot of it's humour and appeal because of the rushed nature of it). At least it did finish.
There are others that I'm sure will come to me as I go along.
Oh and Wez's missus reminded me of this:
Spaced