The most popular tabletop RPG is easily Dungeons & Dragons. Along with being one of the oldest games in the business, it being used in everything from Critical Role to Stranger Things (not to mention a big budget movie with massive names attached) has catapulted it even further into the public eye.
That's not to say it's necessarily the 'best' for everyone, though. There are plenty of alternatives out there, and they excel in areas that D&D doesn't always do as well.