What are your favorites to play on? Anything valid, be it a published RPG setting, an adaptation you did of other media (book, game, movie), a mashup you or someone else create, etc.
GURPS is what I’ve run before.
Always wanted to run Skyrealms of Jorune after playing the computer game, but it remains in the box.
GURPS is what I run now. The Space and Ultra-tech books really add a lot of depth
Numenera written by Monte Cook, its Earth but a stunning nine civilizations (only some were human) rose from the stone age to technological peak before collapsing again one after another. This results in so much unexplained tech it may as well be magic. Magic users are simply manipulating the nanobots in the air for example. Very fun setting with a great world map.
I enjoy high fantasy with a touch of post apocalyptic sci-fi. I ran a Palladium Fantasy RPG campaign set in post apocalypse Robotech. Basically it was my version of Rifts, which is another setting I greatly enjoy.
But in the end I think I will almost always find Star Wars to be my favorite. I have made my own systems that are outside the Empire/Republic, my own crime rings. I have exclusively played the WEG d6 version, but am going to give the EDGE system a try after my D&D campaign is done.
Putting aside game system and mechanics, I enjoy Traveller, Numenera, and Shadowrun, sure. But none of those can compete with Star Wars as a sci-fi setting.
Edit: Honorable mention to Star Trek.
I really want to do a game that’s like a riff on Shadowrun. Near future transhumanism, magic, and the players go and blow up a board room full of oil executives or health insurance leadership or what have you.
I really keep trying to shoehorn sci fi into D&D 5e but it never really works, much to my annoyance. Particularly, trying to get people to learn additional systems is hard, so I keep trying and failing.
Have you heard about SW5E?
Yeah, it’s too space wizard for me :/ want something more like The Expanse but with 5e rules.
Traveller, or better yet 2300AD or Hostile for better fit on the setting with the same classic 2d6 rules. Cy_Borg or Stars Without Number for OSR.
Diaspora, Mindjammer, or Nova Praxis if you’re willing to try a more modern system (Fate), but the setting should be pretty close on them, depending on where in the story you want it to be. Diaspora should be a good fit if you want to explore different systems throughout a network of spheres.
Google threw this at me… haven’t read it, but at least it costs $0 to check it out:
https://www.drivethrurpg.com/en/product/220027/esper-genesis-basic-rules