You mean you don't own it?!

So I'm greatly enjoying life with my iPod these days, and slowly getting our music collection ripped to the hard drive. As I'm going through this, I'm feeling a distinct need for new music. So, I come to you, people who know music I don't.

What one album would you be appalled/afraid/disturbed to discover is not in my music collection? It doesn't have to be recent, just something that you can't imagine not owning.