There are quite a few Black Sabbath, Iron Maiden, and Judas Priest albums that would be considered better Heavy Metal than Metallica’s ‘Master of Puppets’.
By the time Metallica came on the scene, Metal had branched out.
Will agree that Sin after Sin and Stained Class can vie for the title as #2. Can’t think of a Maiden album that has the consistency of MOP. “Number of the Beast” is the only Maiden album I can listen to all the way through.