For me it's Chrono Trigger. I always want to play it. I want to show it to my children. I hope it will be regarded as a masterpiece for generations to come.
I honestly feel a little disappointed that I scrolled this far and nobody mentioned Age of Empires II: The Age of Kings (1999).
This is an RTS game, which is a dying genre. It's also a 24 year old game, which after its release two more Age of Empires games have been released as well, and the game itself has been remastered recently. Yet people continue to play the original game to this day, the multiplayer scene and competitions are still active.
Community consensus is that AoE 2’s mechanics are an improvement on its predecessor, but I personally have a weak spot for AoE 1: Rise of Rome expansion. Fewer things to manage, beautiful wonder structures, and cheap axe man hordes.
My dad and I used to play against each other over LAN. Hearing him shouting and cursing from the other room while I razed his city with scythe chariots is such a happy memory. Man, I miss him. To the dads here, play AoE with your kids.
Second this. Rise of rome, from that unforgettable title music, that start screen, wololo, everything about it tickles my pleasure centers. Have very fond memories of this game.