I think historical significance is less important than actual play experience for the capsule. People can just read about games like Space War if they need to, or recreate them easily based on concept. Compare that to something like Skyrim, where you'd never feasibly be able to identically make it again. I'll also say that MMOs are created by the players. If you moved the game into the future, it would almost by definition become a different game than you tried to put in. That being said, here is my list ... I think (obviously, this is decently impossible

)
Tetris - as Susan said: elegance, impact. Also, local mutliplayer, cross platform
StarCraft - rise of e-sporting, strategy not based on puzzle, but reaction time, network and LAN multiplayer
Grim Fandango - wonderful story, humor, and extremely accessible
Half-Life 2 - shows the idea of progression in sequels, the epitome of physics and FPS gameplay at the same time
Finally, I hate to add this because it's so recent, but Skyrim - extremely complex, shows open world, RPG, and finally, if the world ended today, its the best example of how far we actually came before blowing ourselves up.
Also, the video is blank for the podcast ... so, might be the perfect chance to just put up your favorite submitted animal picture as a placeholder while you guys are talking. Just sayin'.
Another great podcast guys, keep them coming!