I'm going to nominate the first Super Mario Bros.
To me, the things that seperate a great game from a merely good game are longevity and influence. SMB practically defined the 2d platform genre, its seen several remakes and the original still stands up. When you've seen that many remakes and the only changes are the save system (minor, in a game you can beat in <10 minutes if you know what you're doing) you know you're onto a winner.
Course, I'm going to fan the flames and disagree with FFVII being a great game. Its a good if somewhat typical RPG but its aged badly. Not so much the storyline (though half the reason it needed so much supporting material was that the initial translation was dodgy as fuck and it made the storytelling mistake of putting vital plot info in optional scenes) but the gameplay itself. The real differences between it and every other JRPG in the style - the materia system and the limit breaks - were poorly balanced and very much showed that they needed further refinement.
