Amusingly, I’m almost tempted to say Numenera again. Numenera definitely scratches my sci-fi itch, but it’s embracing of fantasy trope to deliver a sci-fi feel makes it feel more fantasy than sci-fi to me.
Despite me liking sci-fi a lot more than fantasy, I’ve actually played very few sci-fi games. Shadowrun is an obvious one I’ve played, but the fantasy elements kind of disqualify it, even though really it’s sort of the opposite of Numenera - it’s fantasy elements delivered using sci-fi cyberpunk tropes.
Starwars is another one I’ve played, but equally that is also a mash-up of fantasy and science-fiction (though I suppose it’s more heavy on the sci-fi than the fantasy). The West End Games D6 system we used is by far one of the most simple, easy to use and flexible set of game mechanics I’ve played with though, so bonus points there!
Hmm, discounting all of the above, I don’t think I’ve ever played a pure sci-fi game. I’m currently running a (mostly) hard sci-fi play-by-post game, but it’s a bit early days for me to call it my favourite. I did (a fair while ago) play a Microscope game where we generated an interesting sci-fi history. Microscope itself isn’t inherently sci-fi though.
I guess out of everything I’ve actually played, West End Games’ Starwars has to be my favourite. The system is elegant and flexible and I’ve had a lot of fun Starwars games. One character was a kid that, through an accidentally incredibly powerful blaster shot (keep those sixes rolling!), almost immediately went to the dark side after killing a guy he only meant to lightly wound. Another was an Ortolan, ex-Jedi chef that reluctantly got involved with a group of rebels. And in one game, our group may have accidentally blown up an entire planet by detonating bombs in a mine, a mine that just happened to be tapping a planet spanning vein of high-energy force crystals as used for light sabres. Fun times!