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.
Mega Man X has aged so fucking well. The controls still feel so tight. There are hidden secrets in the levels, and the ultimate secret if you find all of them.
This was the first game.i ever played, so I'm definitely biased, but this is still probably my favorite game of all time.