farscape and firefly will always be my top two and even the amazing new battlestar galactica can't beat them.
more recent: jericho
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Too many to mention some of my favourites include: Doctor Who, Stargate Atlantis,Quantum Leap,time tunnel,survivors,Torchwood,Babylon 5, Battlestar Galactica, and many more.
Isn't it so hard to decide a favorite? I've got to say all time is The X-Files. I have all 9 seasons and both movies on dvd, and i hope so much when my 4 year old daughter becomes a teenager that I can share them with her!
That being said my first sci-fi loves were Dr. Who and Star Trek TNG. Sliders was great too! Stargate (movie, series(both of them)), DS9 was great too, and I bet i would like Voyager and Enterprise more if I gave them more time! I could go on for hours, so I better stop!
Remember Millenium?