Alternity by
TSR (now
Wizards of the Coast) is currently my
favorite RPG, it's the only one I
play anymore. It's
science fiction genre, and it's universal d20 skill-based
game mechanic is wonderfully simple, efficient, and easy to learn.
There is no single set game world, as with other RPGs, but the mechanic can be applied to any science fiction setting, especially one you make up yourself. Wizards has published several settings: Star Drive, Dark Matter, Gamma World, and Starcraft.
Unfortunatly, Wizards decided in 2000 to stop publishing the Alternity line, due to low sales.