Wow! Only ONE person mentioned Doctor Who in this whole thread and that was only as an "honorable mention?" You folks need to turn in your Geek Cards!
Okay... my top five:
1. Doctor Who
2. Firefly
3. LOST (probationary and only if you forget the final season)
4. Arrested Development
5. The IT Crowd
Lots of folks put up some really great shows on their lists that I haven't thought about in years but were really great shows in their time.
If you love Arrested Development you really need to check out Soap.
Barney Miller, Cheers and 24 were really good series also.
And M.A.S.H. - that was great television there.
@Fetch Questman; I haven't seen Mad Men. I've heard lots of good about it so maybe one of these days I'll find the time to get in on it. I just gotta have time to take it in from Season one episode one until the present. I hate entering a series in the middle of things.