The two games I currently enjoy the most are Fantasy Flight’s Star Wars (Age of Rebellion specifically) and Stars Without Number.
If you like the setting, FF Star Wars is cool if you have the disposable income, because even the core rulebook isn’t exactly cheap and you’re likely going to want at least one additional set of their completely custom dice. The game itself is not perfect, but a lot of fun, with the so-called “narrative dice” bringing a lot of variation to skill check results: it’s completely possible that the dice dictate that you totally succeed at what you’re trying to do, but also have something terrible happen, or the other way around.
Stars Without Number on the other hand has a completely free version that has everything you need. There is a paid version of the core rules that has a few additional (but 100% non-essential) chapters, plus several supplements, but the free edition really is a full package.
The setting is a kind of post-post-apocalypse, with humanity and other alien species starting to come back from a psychic Scream that effectively disabled most of their technology.
The game is very focused on sandbox gaming, giving the GM lots of tools to facilitate this, mostly in the form of random tables and structural advice. The actual mechanics are relatively lightweight and purposely old-school so that it’s easy to import stats for things like D&D Monsters and reskin them to fit the sci-fi setting.
If you’re interested in this system, a revised edition has recently kickstarted and the beta version of the new rules is also available for free. I like the changes a LOT, especially character creation gives players way more tools to make their characters mechanically distinct.