For the relevant games... 20 years is perhaps a bit of a long time, but I only see Galaxy as a timeless classic. The game has tight enough controls that it will be enjoyable for a long long time and the game mechanism are unique enough that it will stay so for a long time too. I guess Portal could fit in here too as the game style is pretty timeless...
I can see a refreshed version of these in 10-15 years as making kids smile.
Games that made an inpact are a lot more numerous, but will definitely not be played by our kids: gears (gameplay), uncharted (redefined TR type games), Mass Effect (WRPG made popular (or is that Oblivion's claim)), CoD, Batman, SSFIV, Crisis....
Heck there are very few games that stand this test of time... Mario (2D), Sonic (2D), Metroid (2D), Zelda (2D), Tetris, Countrerstrike (simple gameplay and controls help) and that is much harder with 3D(polygonal) and the advances that are made due to system capacity both in graphics and the inherent physics of the game (HL2 was a physical evolution as well as a graphical one... Portal will reap the benefits of it, but HL2... only made an impact). Heck I can't find any other game that would be played if not because of the lag handheld have had to consoles (crono trigger would not be played and the DS remakes of DQ and FF are improved versions to make them accessible to non nostalgia prone gamers). There's no secret that my list is mainly 2D... graphics don't matter as much as gameplay in their age.
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