Supernatural, probably the best show ever =)
Heroes, it was awesome first 1-2 seasons... now it's getting messy
24, well the serie is good, but how many seasons can they keep doing the "same" thing?
LOST, last season coming up
Stargate SG-1, ended now, but it's one of the best shows ever.. excellent writers & actors, it got action,
humour and what most other Sci-Fi shows miss - some kind of.. "attachment" to the present reality which is kinda cool.
Stargate Atlantis, same as ^ just not AS good.
Terminator the sarah connor chronicles, too bad it got canceled after the second season, just at the point everyone had been kinda waiting for through the serie --> ended with a huge cliffhanger...
Firefly, cool serie too bad only one season was made (with a movie a couple of years later wrapping up the story)
Jericho, great show.