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.
Diablo
The first multi-player lobby over battlenet. Epic lore, great gameplay, created an entire genre.
Roguelike games are named after Rogue, not Diablo.
But Diablo took it and made it real time, creating hack and slash action rpgs. I’ve never heard anyone refer to Path of Exile or similar as a roguelike. Diablo’s development might have started as a roguelike but making it real time turned it into something new.
Usually Diablo and Diablo-likes are called ARPGs, a term that originally just meant “Action RPG” but is now mainly used to describe games similar to Diablo. Isometric, real-time, character builder RPGs.
I would counter & say Diablo 2 is the better game for expanding on everything that the original did & making it better by adding more.
Diablo is also the first game I can think of with a paid expansion. Hellfire was a 3rd party created add-on to put more content into the game. If you want to talk about creating a “genre”, that’s the conversation to have.