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.
Metal Gear Solid. Every stealth game that has come out since has borrowed something from MGS. Also it is the first game I can really remember that nailed that cinematic video game feel we see so often these days.
MGS also brought us what I lovingly refer to as the "Metal Gear Solid Voice Acting Benchmark."
Is your big budget first party title not voice acted? Why the fuck not? Konami managed to make every single line of dialog in Metal Gear Solid voice acted, without exception, apparently by hiring randos off the street if they had to. So what's your excuse?
Absolutely! The voice acting was like nothing else at the time! MGS totally raised the bar and changed the landscape for what we come to expect out of a game. It's a true masterpiece.