What is the game that is so common to the otaku experience that nearly every fan can relate to it? Not just something so old that everyone has played it, like Pong, but one that is both interesting yet classic enough that we've all been-there, done that?

Just one game?

I want to say pokemon. Not because it's the love of my life, but almost evey anime fan I know has played at some point in there lives. I'm not sure you can say it's common to the experiance as much as brought people into it though.

Final Fantasy VII and VIII is also a thought.

Sonic and/or Mario, Pokémon, Final Fantasy whatevers, Secret of Mana, DDR, StarFox, the list goes on. If I had to pick one, I'd probably guess that Final Fantasy VII had the biggest influence on the otaku community.

I would argue that Mario 3 is most common to the otaku experience. FF7 is arguably more influencial, but it's not as widely played among fans, but rather just has a very vocal following. Just about everyone has played mario 3 though.

Super Mario Bros and Legend of Zelda. They are also the only two video games whose characters I ate for beakfast in the 1980's.