The other thread was about movies, this one is about TV series. So simple question, your favourite sci-fi tv series, why and some background info :P
Mine must be Star Trek: The Next Generation no doubt, i'm currently downloading them and watching, i was born in -93 so i didn't see it back then, but a tv channel here showed the first 2 seasons at some point and i loved it. Even thought the technology is old and at some places its funny, it's still one of the best scifi series ever.
My second favourite is Quantum Leap, Scott Bakula and Dean Stockwell make an awesome pair, and the series came 2 or so years back. It was the highlight of my TV sunday. Aah, 6 hours straight in front of TV. Althought i didn't like the ending, the series overall was really fun.