A couple of different takes:
1. There can't be one RPG to rule them all. You need to be able to cover all possible tastes, including the taste of "I want to run it right out of the box" so toolboxes aren't the answer.
2. If you look at market share and games being played on virtual tables, D&D 5e is the only possible RPG that is ruling the others, as it has more than any others, and orders of magnitudes more than some.
3. If you look at number of RPGs created from the ruleset, one of d20, Powered by the Apocalypse (PbtA) or Fate are clearly the only RPG concepts to rule them all, because they have the most offspring and therefore show they have "the best genes" for evolving and continuing. (If you count the Forged in the Dark games as ancestors of PbtA is up to you.)
4. All RPGs have come at least conceptually from Chainmail, the first pre-D&D ancestor. Without it, the concept as-we-know-it-in-this-timeline would be very different if it even existed in this format. So it is the one RPG to rule them.
5, LARPing involves the most immersion by bringing the physical in as well, so it's the one RPG to rule them all.