The topic is pretty self explanatory. However, I'd also like to see motivations. Here is my top three:
1) -Firefly
Joss Whedon in space!
The perfect mix of humor, character development and action. Damn you, Fox, for canceling this goldmine.
2) -Battlestar Galactica (the latest)
It started out a little slow, but I can appreciate how dark the series has become. The cheerful cast from the first season has turned into a group traumatized veterans that struggle to keep going. I also like the fact that each season seems to have its own theme. (Season 1: creating a new civilization from scratch. Season 2: relation between the military and the civilian government, etc)
3) -Star Trek Deep DS9
I've never seen a Star Trek series with a better storyline or cast. I really miss Star Trek nowadays. Even Enterprise became enjoyable in the last few seasons.
Honorable mention: Lexx. Which was a great series, but a little too vague for my tastes.