I'd have to vote for Traveller. Its had more versions within its system than any other game I can think of except maybe D&D, and thats excluding things like GURPS Traveller and other sytem versions of it.
It is, quite simply, the D&D of Sci-Fi RPGs- the one by which all others are measured.
I have found some more customizable or flexible (HERO- but it doesn't really count), I have found some better within a particular genre (MechWarrior, CyberPunk, RIFTS), and even one that had some better mechanics (Universe)- but none have been as comprehensive, well supported, or internally consistent.
It is, quite simply, the D&D of Sci-Fi RPGs- the one by which all others are measured.
I have found some more customizable or flexible (HERO- but it doesn't really count), I have found some better within a particular genre (MechWarrior, CyberPunk, RIFTS), and even one that had some better mechanics (Universe)- but none have been as comprehensive, well supported, or internally consistent.