I thought I open a topic like this, cos I like them very much, and I'd like to know what other people think about them.
My favourites are:
Firefly
Stargate
Babylon 5
The 4400
Battlestar Galactica
Heroes
Sliders
Of course I saw some Star Trek too(who's not?

), but not a whole series together. Maybe the best is Deep Space Nine. The first three will be the absolute favourite forever I think. It's so sad that Firefly has so few episodes.
edit: is Highlander(the series with Adrian Paul) counts for sci-fi? Cos I like it too very much.