Kindof like what you said, Pokemon doesnt age. I actually made a play though of leaf green a few months ago. There is nothing better than when you leave pallet town for the first time with your pokemon. I always enjoy that feeling when you starter pokemon reaches level 7 when you get your first attack, and then level 12 when you get your first elemental attack and feel so powerful.
Brave Fencer Musashi is another game I can play once a year forever and it will never get old. This is by far my favorite video game ever made.
Nintendo makes alot of its game pretty timeless, Zelda OoT, all of the core series Mario games.
I also feel that the Crash Bandicoot games are timeless.
Overall I feel that any game that does a good job of creating it's own atmosphere, completely neglecting the trends of it's time will be playable for years to come.
For example: noone will play Madden 2002, or 50 cent bulletproof years from now. Or games that portray a super hero, like Barman for instance, his portrayal changes DRAMATICALLY from decade to decade. Look back on the PS2 animated series batman game and you think 90's, Arkam Asylem will be "old a$s" in about 10 years.
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