Hmm, I'm not sure if I have 5, though I'm a huge sci-fi fan.
1. Star Trek: the Next Generation
2. Star Trek: Voyager
3. LOST
4. Sliders
I've heard good things about Battlestar Galactica, but I haven't watched it yet. I'm trying to take a break from getting addicted to long shows. And BSG would be quite the commitment at this point.
MST3K is great, but I can't call it a favorite of mine since I never choose to watch it.
Flashforward would have been one of the best ever if given just one more season.
Damn I forgot about MST3K! Good call there.
Fringe is awesome but needs to find it's feet still. I think with the 2nd half of S2 they started to do so and I expect some really great storylines this coming season.
I hate the new V whit such a passion you can taste from across the room.
Star Trek (all series)
Fringe
And FlashForward before ABC killed it. I admit the storyline did seem a little "out there" and it probably would have been better as a one-season mini series, but it started to get really good towards the end. Oh well.
Oh, boy. For some reason, I thought I had seen every episode. But then, a couple months ago I checked an episode guide on Wikipedia, and I've actually not seen the majority. From those that I've seen, I don't know that I can pick a favorite, but I can at least name a couple. I really like the one in which Data takes on the persona of Sherlock Holmes, in the holodeck. And then there's one in which Worf defends his recently-passed fathers' honor.