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.
I don't think "greatest game of all time" is really a thing.
As you mention pubg for example it was a simply copy as well.
Does it take originality to the The greatest? Does it take being popular?
Agree. To add to this, I might really like RTS and say star craft is the best game of all time. And you might like action RPGs and say Diablo 2 is. Or whatever else.