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.
This is going to be a weird one considering the graphics, but hear me out, the original Deus Ex. Something about the game just feels so well put together that the graphics take a back seat to the gameplay. There is a reason the community around the game has fought so hard to keep it running on modern hardware.