Strictly speaking, there are very few scifi RPGs. Off the top of my head, just Shock:, Sign in Stranger, Freemarket, and Eclipse Phase definitely qualify, and a few others are arguable: Blue Planet, Albedo, Diaspora, and Khaotic. I’m sure there are others I’m forgetting at the moment, but RPGs, much like movies and TV, very rarely take a serious stab at science fiction—most “science fiction” is really just fantasy with robots and spaceships and rayguns.
But I’m not feeling pedantic, so including the many science fantasy RPGs out there, the one I’ve most enjoyed playing of late is Doctor Who: Adventures in Time and Space. Not just because I’m a huge fan of Doctor Who (though that obviously helps), but because the rules are a good balance of embodying the source material, giving a few bits of crunch to sink your hooks into, and being relatively light-weight.
It’s mostly a fairly traditional RPG, with the GM having the lion’s share of authorial power and players mostly confined to controlling the PCs. Those PCs have attributes and skills and (dis)advantages, plus hero points (plot points? I forget what they’re called). But with just a few deft touches, it really tunes those otherwise-bland mechanics to match the show I love. First, and most important, the game isn’t about combat—there are minimal extra complications/rules if a conflict turns into a fight and there’s the genius “initiative” rules: in any conflict, talkers go first, then movers, then doers, then fighters. So by the time you get to take a shot at someone, they may well have run away or closed the door, or in some other way avoided the fight.
Second, it follows Buffy (the RPG) in giving the characters with fewer capabilities more hero points, which helps to make them more the center of the story and often let them save the day in the end. This is a great balancing mechanism in a traditional-style RPG and, combined with the rest of the rules, makes a Doctor-plus-companions game fun for all concerned.