Fallout 3
It isn't considered the best game in the series, but it was my first real experience with the post-apocalyptic genre — and exploring the wasteland was so good.
This game taught me more about the US than movies or popular culture ever did. Searching every house and mailbox, seeing drive-in cinemas and malls — it was amazing.
And the music, of course. Some of those songs are still in my playlist.