Geez, this is a tough one. I own and formerly played Star Frontiers (Knight Hawks wasn't bad and SF is a reasonable juvenile SF game and did provide at least a concept of tactical combat with chits/maps), Star Trek the RPG, Star Wars, Aftermath (I gotta fondly remember the knockdown chart that said things like "hit, spin 180 degrees, fall to one knee, drop weapon"), Morrow Project, Gamma World, T2K (It's near future SF...), High Colonies (a neat little background), All versions of Traveller, Ringworld (whose only memorable mechanic was for the flashlight laser),
and I'm sure there are others if I took a trip into the archives (basement). Lord knows, I used to buy any new space game that came out at one point.
I think Star Ace and Timelords rank pretty lowly, as does Gamma World (As does classic module Beyond the Barrier Peaks for AD&D which was sort of a Gamma World crossover and old Blackmoor LBB format AD&D book which had some sci-fi stuff IIRC). Rifts looked like something ... avoidable... as did a lot of GURPS (not GT). And similarly, Role (ROLL) Master's Tech Law and Spacemaster were pretty icky. Space 1889 initially seemed very retarded, but in the later days, I've discovered it presented an interesting twist on an interesting time period, and now that I know a bit of the real history predating it slightly, it would hold much more appeal.
I (sadly) missed Metamorphasis Alpha and I've avoided Alternity.