What science fiction RPG have you enjoyed most? Give details.
Like superhero games, this seems a genre that folks are likely to either really be into, or hardly ever play. Unlike superheroes, I'm not quite sure why that is. Sci-fi, I think it's safe to say, held a much bigger place in Western pop culture at least through, arguably, the popularization of fantasy in the early 2000s (what with your Harry Potters and your Lords of the Rings). Certainly, during the formative years of the role-playing game hobby, even though fantasy lit was enjoying a major revival of interest, sci-fi was still the reigning champ. I guess there's something that's just a bit more accessible about fantasy?
At any rate, I certainly fall into the category of folks who gravitated naturally towards fantasy to the near-complete exclusion of sci-fi. I was certainly aware of games like Traveler and Star Wars d6, and they held a certain undeniable appeal, but that appeal never really translated into a burning desire to pick up a game and do something with it.
There were half-hearted attempts here and there (though, even then, they tended to be some sort of science-fantasy affair, like Mutant Chronicles), and one of my long-time friends, Tim, is a huge sci-fi guy and has talked a lot about running something in the sci-fi genre - but he hardly ever GMs, so that's that.
Now, does the cyberpunk genre count as sci-fi? Or should I save it for tomorrow's question about post-apocalyptic games? Nah. Let's call it sci-fi.
So yeah, I played a fair amount of Cyberpunk 2020. Even then, though, I think my group and I were more enamored of the concept of the game rather than actual gameplay. I mean, we never managed anything resembling a campaign. Just one-shots that usually culminated with a big shootout and lots of blood and guts getting spilled everywhere. We made lots of characters and giggled when someone picked up the Mr. Studd Sexual Implant (3d6 Humanity loss!).
Yet the only gaming-related item of clothing in our group was owned by Alex, a t-shirt that featured the above illustration and a garish "Cyberpunk" logo on the front. (Yeah, we were pretty cool.) But certainly, we never really explored the full potential of the game or the genre.
At any rate, the cyberpunk genre still holds tons of appeal for me, especially since nowadays there are kind of two different ways to run it: either as intentionally "80s-retro" sci-fi, or else attempting to adapt the genre to current trans-humanist, post-scarcity visions of futuristic dystopia. (Personally, I tend towards the former, both because that's what I originally fell in love with and, frankly, it's less depressing.)
I'm not sure if CP2020 would be our choice of system, but out of all the different flavors of sci-fi gaming out there, this is the genre that continues to hold the greatest appeal for me.