I don't think this is an easy set to come up with, because I can love a series today, but when I next watch it ten years from now I'll think it's crap, having aged gracelessly and been topped by other series.
Series which I liked, but wouldn't like to re-watch:
1) Twin Peaks. I loved this when it came out. Among the first series which totally hooked me in.
2) Star Trek: TNG. I liked it, but only because it was the only sci-fi out there at the time. I think it's shit now.
3) Babylon 5. Loved it, with its sprawling plot arc, but wouldn't waste my time with it now.
4) OZ.
5) X-Files. I put this grudgingly here, because I hate it now, but I watched it then and I was pretty riveted back in the day. It started a trend which brought a lot of shows dealing with supernatural premises to tv screens, so it played a positive part in tv history.
Other winning series:
1) BSG. The new show. Probably the best show ever.
2) Aeon Flux. I couldn't heap enough praise on this thing.
3) Star Trek: TOS. If you get past the camp, you can see that this series already pretty much did everything all sci-fi has ever done afterwards.
4) Deadwood.
5) Miami Vice.
Some other and more recent honorable mentions:
Carnivale, Sopranos, Jericho, Rome (first season only), Generation Kill, True Blood, Entourage, Top Gear.
On a contrary note, I find Stargate in its various incarnations offensively terrible (except the movie).
Right now, I'm not actively watching any series, but I should probably catch up on True Blood and the new Boardwalk Empire certainly has my interest.