My favorite system of all time is HERO, followed closely in order by Mutants & Masterminds and D20.
However, I voted Classic/Mega Traveller. It was my second RPG, right after D&D, and I haven't seen anything that really captures a hard Sci-Fi feel & flavor quite the same way.
While it came out after Star Wars, it still feels more like Golden Age/Silver Age sci-fi stories by Niven, Bova, Clement, Clark, Heinlein, Asimov & others, as opposed to still later RPGs which have a bit more of a "space opera" feel. Only the defunct SPI's Universe seemed comperable, but that game & company folded long before there could be any kind of accretion of quality supplements.
Not that this is a negative commentary on other sci-fi RPGs. I LIKE Traveller D20, Prime Directive, and several others, and have run sci-fi campaigns in HERO...but Traveller is still the king for me.