1st edition AD&D. It was what turned role-playing into a hobby from a fad. It's the version that brought RPGs to the mass market and made TSR into a force to be reckoned with. It is the D&D of the '80s that dominated the hobby like no other version since it has. It had it's own cartoon series. It had one of the first multi-media marketing campaigns in Dragonlance. It was the gold standard of RPGs for more than a decade.

I might have said 3rd if 1st Ad&D wasn't a necessary (and even more influential in its day) forerunner.