Excluding compilations (greatest hits albums), of which bands do you own every single album they've put out? In my case, even though I have more than 300 cds to my name, most in the hard rock/metal genre, its Black Sabbath. I have every single studio and live album by this legendary band. Close second would be KISS, I've got 'em all except the horrendous Unmasked album which I'm in no hurry to buy since it sucks. I also have almost all of Iron Maiden's catalog, I'm just missing Virtual IX and Maiden Japan.