Any game that you can confidently run and easily explain to a new player, that they find interesting and want to try, makes for a great introductory RPG.
My personal recommendations is to start off with something the new player enjoys. Are they into the Marvel Cinematic Universe? Introduce them to the hobby with a rules-medium Supers game. Do they like Science Fiction? Perhaps they should begin with classic Traveller or Star Trek Adventures. The idea is to get the ball rolling with a title or genre that will appeal to them so part of the hurdle of learning RPGs, the investment of emotion and mental energy, is already taken care of.
It is much easier to teach someone something they want to learn.
I don't recommend starting with Dungeons and Dragons these days because it gives the impression its the only game in town. Those interesting in RPGs are going to hear about or already know D&D so start them off with something else so they get better exposure from the get go.
As noted briefly above, I recommend rules-lite to rules-medium game for the initial introduction. Start simple, let the person getting into gaming process all the components and then gradually add complexity. This is how nearly every hobby works. It also doesn't scare or intimidate a newcomer with have to go through three textbooks and material just to play a game. Let them enjoy themselves first and if they do they'll likely want to dig into the deeper stuff later.