Im not sure there is one at the moment. The longevity and replayability of the game will be seen for years and years. Of all the games out right now, i cant think of a single game that has more of a fun-factor than Diablo 3.
I can list all the reasons why I feel Diablo 3 is such a great game, but I think everyone already knows them at this point.
Do you think there is a better game than Diablo 3? If so, what is it, and why do you feel that way?