I've been playing since the NES and despite being from a low income family I had the luck of being able to play and own many consoles over the 3 decades of my life, plus some pc.
If you ask me right now? Resident Evil 4 (2005).
A before and after in gaming, to this day still extremely fun to play even for casuals but 20 years ago it was THE masterpiece. And everyone took notice of it, everyone played it, even players that didn't cared about resident evil. The gameplay was so good that it got photocopied by everyone right after in the action genre.
Arguably the last big innovator in videogames minus Minecraft and... PUBG (Fortnite did it better I know).
Try to NOT pick your favourite game, that's a different thing.
Super Mario World is my answer to "If someone who never played a video game before asks you to introduce them to gaming, what will you start with?"
Possibly becasue it's a major entry point for me into gaming, but it's also colorful, specifically designed to be beginner and child friendly, it's whimsical, the controls aren't that complicated (though there is a deep end to that pool) and being a 2D platformer it's maybe not nausea inducing to people not used to 3D graphics.