Haha, Heavy Rain. I knew this would be mentioned before I even entered this thread and that's one of the first titles that came to my mind as an example of a game which won't be played in 20 years.
Actually this is a very good question and I think we have to distinugish between relevance and own taste.
Examples of games from recent years that will be called relevant in 2 decades:
World of Warcraft is without a doubt the game that will be remembered as most influential.
Minecraft will be remembered as one of the most successful indie games ever created.
Wii Sports and Wii Fit will be remembered as selling a system like no other games before.
(A counter example: I think for some reason Nintendogs won't be called relevant in 20 years as its influence is already neglected today).
Games that will be fun and played in 20 years from now:
Plenty - as remakes and as as re-releases. I and thousands of other people still play The Secret of Monkey Island more than 20 years after its release and this is just one of many PC games from 1991 or before that are still played today and there's hundreds more on consoles.
The following come to mind:
Starcraft 2, Civilization IV, SMG1+2, NSMBWii/DS, Mario Kart Wii (as a few examples for games which will be considerd as providing timeless fun just as their predecessors - Diablo III will most probably be in this group as well).
Red Dead Redemption (not for its gameplay which isn't unique but for its pretty unique setting), Dark Souls, Valkyria Chronicles, Mass Effect Trilogy, Flower, Portal 1+2 (as a few examples for possible future cult games of this era which will most probably still be played by fans in 2 decades).
