"What science fiction RPG have you enjoyed most? Give details."
Are we talking core ruleset? Or possibly a setting? I suppose that's the difficulty with vague questions like this, you can never really be 100% sure. However, because my favorite science fiction RPG exists in multiple playable formats, I'm going to have to go with Fallout. If you want a game system to go with it, try Savage Fallout. There's also a PnP transcription of the in-game rules, which are themselves based off tabletop.
Perhaps it's unfair to put this here because I haven't actually played Savage Fallout yet. Or Fallout PnP. However, I have played the originals, which, much like Baldur's Gate before them, is a transition of a tabletop game system to a computer. Also, I don't care if it's fair; the lore is rich, the imagery is unique, and the setting is every kind of badass.
It's close to a hundred years after the end of the world and humanity survived. Cities have been razed, oceans have boiled, a radioactive sheen has been draped over the known world. Humans eke out a living doing whatever they can, some have mutated into Ghouls, others have been FEV'd into Super Mutants. There are 1950's sci-fi robots because that's awesome. Everything wants to kill you. The core games take place in Southern California.
Which is my home.
It's like Dark Sun with guns. Which solves that chicken/egg question for me. I like Dark Sun because it's like Fallout with magic. With both systems, the game is survival, and if you're not playing a videogame, being a good guy doesn't pan out very well for all the parties involved. Because GMs can be assholes (I love that I'm writing this on GM's day).
Look, if you don't know what Fallout is, I don't blame you. Just buy it, and play it, and tell me I'm wrong somehow. I won't get mad, because you're wrong.
P.S. Apparently you can't buy the original Fallouts right now because Bethesda sucks. So do that other thing. The one with a jolly roger.