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.
Funny story; when I first heard about the game (2009 or 10, so before really anyone had heard of it) I thought the name was Mind Craft. Luckily I easily found the correct game and started playing. I think rails came out shortly after. The games come a long way. It's barely recognizable now.