I've been watching a lot of older sci-fi shows lately, but mostly Star Trek.
Star Trek
I was not alive when this was originally on TV, but I did watch an episode or two when it was rerun many years later. Watching it now, it's pretty awesome. I guess the quality really drops after the first season, but so far it's really good.
Star Trek: The Next Generation
I'm do remember watching this on TV when it was first broadcast. I went and got the whole series on Blu-ray on a whim, not really expecting to like it, but I ended up loving it. I even liked the first season, which people seem to hate. The Blu-ray set looks amazing, thanks to the fact that the show was originally shot on film.
Star Trek: Deep Space Nine
I actually have a lot of nostalgia for this show; I was not a Star Trek fan back then, but it's a show I'd always end up watching if I happened to catch it on TV. I found the setting, a giant space station complete with a shady bar, to be very intriguing. Watching the series now, it's even better than I remember. I like this sort of gritty Star Trek that's still optimistic.
Babylon 5
I had never seen this back in the day, but got it because I liked ST:DS9 so much and heard it was a lot like it. So far, I'm partway through the first season, and I really don't like it. The characters feel like stereotypes, and the acting is sometimes really poor with ridiculous accents. It's also lacking the fun but thoughtful sci-fi stuff from the best of the Star Trek shows; you get a bunch of assassins and monsters going around the station early on, but not much else. I've heard that the first season is poor but the series gets a lot better from there, so we'll see.
LEXX
I've got some nostalgia for this one, though I barely remember it. I recalled it being a really weird, slightly sexual sci-fi show, and I was interested in revisiting it and actually watching the whole thing. I was surprised to find that the first season is actually four feature-length shows. I'm halfway through that, and so far it's awesome. I really like this bizarre sci-fi stuff, and the fact that all the characters are sort of annoying is both hilarious and endearing. Yes, sometimes the acting is poor, but it's somehow not so annoying in such a weird show. I like how, so far at least, there's basically no exposition at all, and instead you just figure out what the world is like bit by bit as the show goes on. I absolutely love the trial scene in the first episode, which consists of pre-recorded holograms sentencing people to have their organs harvested through hilariously over the top machines consisting of saws and blades in this far future world of AI and space travel. Black humor at its finest.