Hiya.
I'm actually kinda partial to a couple...I'd have to say: Gamma World 3rd Edition, Living Steel, Battletech/Mechwarrior and Aliens.
Now, GW3rd is more of a "science-fantasy" setting, with all the mutations and whatnot, but I just love the "just enough info" presentation of the game to let me decide on the specifics.
With Living Steel, (which, I'd like to point out, I've never actually played), we have the initial promise of humanity reaching out into the stars...only to be thwarted by nuclear armageddon and some crazy "cryo-stasis saviours for the future of the human race". I get very stark, distinct visuals from this game. Me likey!
The Battletech/Mechwarrior setting...vast. It encompasses a HUGE range of potential stories. You could play thirty different campaigns using this setting and every one would be different. You could do everything from "abandoned planet struggle for survival", to palace intrigue to military power struggles, to epic space-battle 'divvying up the galaxy' campaigns. Go pure PC oriented, pure battlemech oriented, or somewhere in between. Clans or no clans. Whatever you want, you can do it.
Lastly, the Aliens setting is one of those "believable" sci-fi settings. The space travel and tech just seem so much more "believable" than the tech from BT or LS (and definitely more than GW). Governments are there to manage the day-to-day living stuff, but the corporations are the ones that make the actual 'big decisions' because they hold all the tech and money. The solar system and little corner of our Milky Way seems so vast...yet so small. I just like the "realistic" feeling of it the most I guess. Oddly enough, in our campaign (we've only had one, and we always use it when we play), I don't think we ever actually ever encountered an "Alien"; closest we has was one crazy PC xenobiologist who obtained an "egg" and was secretly transporting it back to his lab...hehe...yeah, that didn't end well...
^_^
Paul L. Ming