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.
Final Fantasy 1 - It wasn’t the first RPG, but it pretty much defined the series. It still has tons of playability, I revisit it more or less every 5 years. I still have yet to beat Warmech, and only have encountered him a handful of times.
But most of all, it’s the game that saved Squaresoft. If it had failed, we would have missed out on so many great games, including ones also mentioned in this post.
Runners up have to be Donkey Kong, which brought us Mario, which in turn restored vitality into the at home console game industry, and Double Dragon, which brought us PVP and Co-op combat.
Honorable mention would have to be that Simpsons arcade game where Marge can fight with the vacuum cleaner and TMNT 2 - Two classic, very difficult, drain your change jar games. I’d throw Mega man 2 into that mix as well.