I have to choose 1 game for this generation: That's how I'll do it.
Witcher 3.
Runner ups: Breath of the Wild, Bloodborne and The Last of Us.
Reasons why not:
BotW: It's basically there, though I go back and forth about it. I think popularly there's no way it isn't a classic.
Bloodborne: It's amazing but too iterative on Souls. It's basically just Souls it just established a new IP with a perfect theme and story, but it's still too familiar.
The Last of Us: A hallmark entry in writing/acting for video games but ultimately it falls short as a "classic" game because it's so... basic, really.
Why Witcher 3:
It iterates on certain RPG trends as well, takes inspiration from Red Dead Redemption and Mass Effect for its approach to open world and dialogue-choice story. It takes the prize for me because it systematized its world just right, with emphasis on narrative functionality and narrative interaction the most. You'll want to slay things but not so much that you won't also want to dabble around a city just finding people to talk to. The tone is straight off the page of a Witcher book, which is an achievement - Tone, while not 100% consistent manages itself extremely well for a 100+ hour game and just the insistence on production value for all conversations and the amount that there is is breathtaking, to quote Keanu Reeves. It's basically Mass Effect 1, a game that also impressed me with hours and hours of simply dialogue between NPCs that you engage with which goes beyond run-of-the-mill fantasy/sci-fi tropes, and actually stimulates my intelligence. It's that but on the scale of Skyrim and that is so impressive that it makes you question how they even did it (the answer being enslaving their developers, polish currency differences and other unfortunately un-admirable results). I think this game already left its mark when you look at the last two Assassin's Creed, the failed Mass Effect Andromeda (which did actually use its latter year to copycat Witcher design) and most likely many other ambitious releases... but none of them beats out Witcher 3 because they don't have the same narrative intent and tonal consistency or the level of empathy and intellect to the writing.
It's not a perfect game but it is a jaw-dropping kind of game once you realize its scope. It's a game I beat twice with about 60 hours of gameplay, and not only did I see the main story unfold certain beats differently but I saw different side-quests and I also haven't seen all side-quests, and I'm so okay with that, because each one asks you to indulge yourself to hear its plea, and it doesn't feel like a game that exists in the service of Geralt as the player. It's un-videogamey in the most complimentary way possible.