Alien and Coriolis are very similar - same company, same core engine. Alien can be played quite easily without using the eponymous Aliens; at its heart, it's JUST a sci-fi game with a strong system for stress causing PTSD.
Coriolis is very much a Chronicles of Riddick feel in the corebook look and setting.
Aliens has smoothed out the mechanics a good bit.
Star Trek Adventures has a couple of flaws due to 2d20 that the designers ignore, but otherwise is a pretty solid Gamist-narrativist hybrid. Character gen has 3 basic modes - Random Lifepath, Chosen Lifepath, and Pick as you Play mode. The two lifepath modes can be mixed easily, as it's roll or pick at each step. It is, however, strongly tied to Star Trek.
Firefly is years out of print now, and the PDFs came down last month. Otherwise, I'd recommend it.
Genesys is the Star Wars engine as a generic game. There are two sci-fi settings and more to come, supposedly, for it. THe dice are marked with different symbols, but the same names for the symbols, and the same distributions and colors, so Star Wars dice can be used with Genesys.
Mongoose Traveller (either edition) or Cepheus Engine are excellent choices as long as you don't want fidelity to the OTU as Marc Miller sees it. T5 is not, however, easy for many to wrap their brains around, but is the official framework going forward for Marc's view of the OTU. (But you have to extrapolate a lot from the tables until the M1900 book.)
Mongoose also has done some OTU stuff, canonistas like me cringe at them.
Mongoose has done far, far better by 2300 as a setting. Their hired author is excellent, and a deep fan of 2300.
DUNE is due out in 2020-Q4 or 2021-Q1. Also 2d20. Can't say much more than that - NDA. I will say: I am enjoying the playtest.