Generally I don't like top 10 metal lists unless they're sub-genre specific. Are you analyzing albums in terms of how they hold up today? Are you giving consideration to the influence they had on future bands/albums? Either way, it's really hard to say album x is "better" than album y. Then you have fans, specifically of metal, who have issues with whether or not album z is even metal in the first place, due to there not being a clear line between metal and hard rock or whatever.
Most top 10 all time lists are full of Sabbath, Maiden, Priest, Slayer, and Metallica, with multiple albums from these bands in the top 10/15, which just seems masturbatory to me. Yeah, those bands made some great and influential music, but really, did any of their albums do anything earth-shattering that their other albums didn't touch on? Definitely needs to be a rule for not including the same band more than once unless you can point to two or more of their albums and see them as being truly unique.
Quite the task you've put upon yourself, Metalhead