I was just browsing through my RPG collection last night looking at the few Sci-Fi based RPGs that I have.
The original Mekton by Mike Pondsmith, the life path system for generating your background was the coolest!
Deluxe Traveller, the original rules, but in full 8 1/2 x 11 sized books as opposed to the mini books. My favorite edition.
Darwin's World, fairly new... came out post D&D 3.0 and was the first attempt at 'Gamma World' for d20.
Serenity, based upon the movie and Firefly TV series... it is beautifully written but I am not sure the rules system is for me.
Alternity, created by TSR just before they were acquired by WOTC. It really looks to be a great scifi rpg... but I never got to play it.
Dreampark by Mike Pondsmith, simple system... versatile... this would be a great game to introduce new people to role-playing.
Buck Rogers XXV by Mike Pondsmith (again)... TSR published, used the AD&D rules as a basis. Well done. Easy to teach. Accessible to D&Ders... very good game. Highly recommended.
StarWars by Greg Costikyan - original edition published by Westend Games. The beginning of the d6 system. Great game!
So, what Scifi RPGs do you own? Or better still, actually play?