Some developers make an awesome graphic like Crisis, some make cool stories like Gothic, some have awesome sound, some of them have cool game play like Angry Birds. But lets think in project of time. Any graphics will be improved with time. Like 3D graphics of 90th is much worse then 2010th. Same with game play. With time we will have cool things like physics or destroying everything or burning in fire. Lets be honest Angry Birds will suck without cool physics. I think there are 3 things witch will be less affected by the time:
Story, Characters. Music.
So I think this parts of the game should be the best to make game classic. But it's just my opinion.