In terms of longevity and flexibility, Traveller is the best SFRPG, IMO (although that may not be the question you're asking). Star Frontiers was for Battlestar Galactica chumps (sorry fans), Space Master was Role Master with tech (in other words, fine if you liked RM), Space Opera (Fantasy Games Unlimited) was pure pulp (qv Star Frontiers), Shadowrun was science fantasy as opposed to SF (nicely done though). I can't speak about Alternity; I know too little about it. Ditto Dragonstar. There are (or were), of course, Star Trek and and Star Wars RPGs. Their titles speak for themselves.
Oh... Chaosium's Ringworld. Great game based on Larry Niven's Known Space mythos. It was supported by a single supplement, The Ringworld Companion. Sadly, it was as short lived as the exotic debris from an atom-smash.
If you'll excuse me, I'm going to fondly remember Skyrealms of Jorune for a while.