I wish they'd bring back a good Stargate. I liked SG-1 and Atlantis was ok, but they lost me with SG-Universe.
Firefly is an obvious one for most those familiar with it (dummies at Fox)
I know some here were so-so on it, but I thought Being Human was decent. Better than other stuff that's still around. (Fox strike again).
Highlander was alright, but it probably hasn't aged well. The spin-off "Raven" was a mistake imo... they should have gone with a spin-off based on Methos. Seemed pretty glaringly obvious to me that would have been a gold mine of possibilities. (For those who don't know Methos was the oldest living immortal and was, at one time, one of the Four Horseman per the TV show storyline - course, they may have had to get past his pacifist streak if they did a lot of modern day episodes, but that wouldn't have been hard...).
Rome on HBO was good. Not quite sure why they ditched that. Could be problems caused by how the first season went down (i.e. slow ratings at first, didn't hire everyone back right away, then the show got popular and they had to scramble to get their cast back, etc). I was also told by one of the HBO reps it was very expensive.
Battlestar Galactica I liked at first too. But it got to the point where I just wanted to slit my wrists watching it due to the depressing episode after episode and I stopped watching it altogether as a result. So I guess you could say I'd like to see it done better.
I also wish Sci-Fi would take the money they spend on those awful movies they make and do a new series. They usually tend to do a good job on their series. Movies? Not so much. If the name "Roger Coreman" (or something like that) is in the credits you know it's going to be over the top cheesy.