I know this is usually typical nerdbait, but Chrono Trigger. I remember staying up until 5-6am on a weekend as a 4th grader because I never played a game so engrossing before.
I never realized what a huge gamer touchstone it was until I was in my 20s, because I just remembered it as that time travel game I played once. Now I've bought it and replayed it on almost every platform it's come out on.