I will give the kind of answer I think this is aiming to elicit in a minute, but first I want to give what I think is the best answer.
The best introductory RPG is the one your friend(s) already playing RPGs is currently playing. The single best way to learn RPGs is not from a book (although that is how I learned long ago, in the before times), but from friends around a table engaged in a game.
The second best is whatever event is listed as “new player” friendly at your local gaming, science fiction, or fandom convention’s game room. It has most of the “learning from people with experience while playing” advantages of playing with friends, but lacks the long-term nature and built-in social connection.
The first and a half, coming between the friend’s game and a convention game is a regular game store game, often done through the organized play program of a publisher. Wizards of the Coast has Adventurers League for Dungeons & Dragons, Paizo has The Pathfinder Society for Pathfinder and The Starfinder Society for Starfinder, Chaosium has Cults of Chaos for all their RPGs plus two board games, and Monte Cook Games has Cypher Play Op for their various games using The Cypher System.
The organized play game has the potential for a social network like the friend’s game. In fact, it might introduce you to the larger hobby better as some of those games played by friends might be a group that plays one system and only with each other. However, you have to do the work of making friends and contacts. It is also less likely someone will do a lot of handholding to keep you in the game as they don’t have a personal investment in having you in the game.
Now, onto what I think this question is about, the best system for beginners. While it is not my favorite edition of the game at this point, hands down the best introductory RPG is the current version of the Dungeons & Dragons Starter Kit. Note, this is true today, it was true in 1980 with the classic Red Book Basic. It was true in the 90s with the Black Box Basic. It was true with the various Third Edition starter kits. It was true with the Blue Book of Dr. Holmes that was my entry into the hobby. It was even true with the nostalgia-themed boxed set for Fourth Edition.
It will be true in 2042 when D&D is on its eighth or ninth edition.
The reason the current D&D starter set is always the best introductory game is twofold. The obvious reason is it is designed for newcomers to the hobby. It is not the only game that does this, though. Paizo has similar kits for Pathfinder and Starfinder. Fantasy Flight Games had them for their various Star Wars RPGs. Plenty more kits like this exist.
What the D&D starter set, whatever edition, will always have over the others is D&D. In two years we have the fiftieth anniversary of D&D and the hobby. Even now, after this time, outside of the hobby “roleplaying game” and “Dungeons & Dragons” are synonymous. Every serious role-player should be familiar with D&D because it is the lingua franca of the hobby and the common standard against which other games are mentioned. It remains the most common first game and, except possibly in the mid-90s, always has been. Even then, while the various World of Darkness games might have been an entry point for more people that year, people from 1974 until the 1990s who started on D&D overwhelmed the numbers brought in by Vampire or Werewolf.
Every player should know one version of D&D and probably two, one from when they entered the hobby and the current one.
The Dungeons & Dragons Starter Kit is the best introductory game because it is designed for a beginner to teach themselves the game, and because it, by what it is, brings them into the mainstream of the hobby and the long thread of its history.