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.
John Carmack’s lighting and raytracing code is what catapulted the FPS genre forward; without Doom/id there is no Quake engine and with no Quake engine (or the iterations thereof) you’re missing the core component of 90% of shooters for the next decade.
Someone else could have built it eventually, but Carmack just laying down this crisp and functional framework and licensing it out to everyone to use in their own games was a huge step in comparison to what would have otherwise been a hundred isolated game devs trying to implement good lighting engines on their own.