I just finished Battlestar Galactica: Season 2.5.
I could probably write pages about everything that rules about this show, but I won't. You either watch it and don't need me or you don't and won't care.
The most important thing that makes it exceptional, though, is the fact that they never just rest on the last cool thing they did. They consistently shake up the goings on. They consistently require us to reconsider the meaning of what came before.
In fact, I keep half-heartedly assuming they will continue in one direction and keep being surprised by how very much they're willing to keep things moving and changing.
Outside shorter run shows on the BBC and HBO, I can't think of a better series... in the last 15-20 years, at least.