Lady Blackbird is going to rate a lot of solid mentions here, but not from me. Lady Blackbird is a fun, great game, but very stereotyped and limited in scope. My choice is a little sci-fi game that I have played a lot of and really like. A tiny game fit in the least amount of space offering a universe of possibilities.
RPG: Edge of Space
This RPG from Matt Jackson of Chubby Monster Games is a brilliant sci-fi game aimed at action. The blurb really makes it sound like that it is all about Space Marines killing stuff. You can certainly play it like that but I have played many a game with subtlety and finesse the focus. Rules lite the game allows you to launch your space campaign in minutes. Character generation is simple and quick. The feel of the game speaks directly to a solid science fiction setting in the least amount of space.
I was lucky enough to get some of the special booklets that Matt Jackson made by hand several years ago. This game is tiny and awesome. These two books are my most prized RPG possessions. This game has everything you need for a full on sci-fi campaign. The rules act as a scaffolding that you can build off of. Perfect for a universe that you envision as opposed to a universe that comes with its own conventions you need to adapt to.
Check this game out. It is $1 USD on Drive Thru and worth every cent. I know that Matt has a second edition rolling around somewhere as I did a read of it a couple of years ago. I hope that this post may spur him on to release that soon! Have an awesome day and keep rolling!