I've been thinking about this for a while and also had a long fun chat with a friend discussing it, so I thought I'd bring the topic to the forum too.
After some hard pondering, I came up with these ultimate favourites:
- All time greatest for me has to be Buffy the vampire slayer. Probably sounds rediculous to most of you and I myself initially put it of as a piece of teenage masscrud, but as the series went on and I catched a few more episodes from the later seasons, I soon found myself hooked and realized it was genious. This show has the best plot twists ever (the show's premise allows the creators imagination to run pretty wild), the characters are very well developed and the occasional humor is dark, witty and fun. It doesn't hurt that Sarah Michelle is pretty hot too.

- The best animeseries that I've watched is without a doubt Azumanga Daioh. Before seeing the show I couldn't have imagined that I'd praise an anime about the everyday life of japanese schoolgirls (without the usual blood, sex or giant robots), but now I've seen the light.

- The true eye-opener, when it comes to comedy was definately Southpark. A cartoon that made me realize how twisted my sense of humor actually was.

- The only space sci-fi I ever bothered to follow and truly enjoyed, was LEXX. The idea of basing a show on the premise of an ugly, horny guy who can't get laid, destined to travel in space (on a spaceship that eats planets) with a hot chick that's not interested in him in the slightest plus a lovesick robothead, is so insanely absurd that it's brilliant. And they pull it off too. Shame that I never saw how it ended, but I think the WWW can help me with that.
- Even though I don't really follow soap operas, being a guy and all, I admit having really been into Melrose Place when I was younger. The plot twists are great and plentiful.
- Reality shows aren't really my cuppa, but I gotta give respect to both Survivor and the Amazing race, which for me are the only two shows that justify the existence of the otherwise crappy format. Still I'm not quite sure if I should add even these two to the list, but as I'm doing this mainly in genres, these are definately the pick of the litter in their's.
There are a lot of other very good series, but I really tried to only put the very best ones IMO here. Also some series, like Lost for example, I haven't seen far enough to decide whether they belong here. I'm also watching X-files (they're showing reruns) for the first time now, which seems like a strong future candidate.
(Note, I'm leaving a lot of stuff, mostly cartoons, that I watched as a kid out, since I don't remember them well enough plus we already had a childhood cartoons talk a while ago on the old board.)