Metallica

So Metallica's album Master Of Puppets was ranked on the internet as the greatest metal album ever.
So my boyfriend asked me is M O P the greatest album in my opinion? My response:
Well this is such a hard question.! I don't know, because if I was to pick the greatest album ever, I'd obviously pick one of Metallica's albums, but I'm not sure which one. M O P is a great album, definitely on of their greatest but I don't know if it is the best. I obviously wouldn't pick St.Anger but i think people should appreciate it more, considering how much effort it cost the band to put that album together....