Rules: just pick 1 and explain why.
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.
RuneScape
I don’t personally play it anymore but it having still players to this day and thriving proves it.
Edit: On second thought it’s probably Java Edition Minecraft. Thing spawn an industry around it. Now people’s livelyhood depend on it. A generation learning java programming just to make plugins or mods. I bet this also what increase the number of java developers. Probably inadvertently also introduced to Blender. Being accessable and sandbox made that possible.