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 think an interesting way to define the greatest game of all time would be a game that captures your attention the longest and provides the largest variety of ways to play.
To that end, I would say StarCraft 1.
My personal reasoning is I love games that have user generated content to keep you coming back to enjoy fresh experiences. StarCraft 1 had a game mode called use map settings where you could play maps made using the built-in level editor. There was enough complexity in the level editor that basically the sky was the limit.
You want to play a tower defense? StarCraft 1.
You want to play an RPG? Which franchise do you want to play, cause a lot of them are on StarCraft 1.
You want to play an action adventure game? StarCraft 1.
The list goes on lol
I've never personally played it, but I feel like another example of this would be Roblox and another one that I have would be Gary's mod.