Sure, I can run D&D from memory. Okay, I love to tinker with Talislanta evolving skill and magic systems. Yes, TWERPS is the "The World's Easiest Role-Playing System and RISUS is the simplest.
But if I had only one RPG, I would give that honor to Call of Cthulhu 6th Edition.
It took many years to move over from sword and sorcery gaming to Lovecraftian horror, but I think that was needed. CoC is not D&D in the 20's. Combat is rarely needed, and when it is, it is terrifyingly brutal. Investigation solves many of the world's secrets man was not meant to know, and when it's doesn't, a PC sacrifice or six of the highest order won't necessarily stop the oncoming terror, but it will delay it, for another group to attempt to delay it further.
While combat can sometimes get bogged down in minutiae, the skill system is a percentile masterpiece. The skill advancement system rewards those who try and try again, and I can wrap my head around ability checks of Conx5 or Strx2 rather than a set number in a system such d6 Star Wars.
I haven't even run the system for a 1/3 of my gaming life, but I can easily see me teaching potential gamers how to role-play using CoC, rather than the current incarnation of D&D.