Well, that implies a single answer, and that excludes all sorts of "...at this or that" qualifiers, which means a candidate would have to be at least applicable for every possible use or permutation or desired quality, goal or agenda of RPGing.What's the best RPG?
If we're not charitable, and don't consider the whole range of a 'core system' like d20 or BRP to be a single game, that excludes virtually every RPG ever made.
The remaining contenders might arguably be FUDGE or Hero/Fuzion or something.
I'd go with Hero System, obviously, probably the first universal version: the 4th-ed BBB/HSR.
If we are charitable, d20, BRP, d6, d10/interlock, Fate, PbtA, Savage Worlds, Gumshoe, GURPS and I'm sure quite a few other core systems or multi-genre or even just highly-adaptable RPGs might be considered.