I rarely listen to Sabbath, Maiden and Priest any more.
Whilst they have some excellent material scattered amongst their extensive back catalogues, most of it is either jaded by regression into rock technique, attitude and composition (both Maiden and Priest are excessively camp at the best of times). I guess it works OK as semi-background, social or whatever music with a dash of intelligence to escape the cycle of Floyd, Zeppelin and what else was popular in the 70s.
Bands such as Satan (UK), Angel Witch, Blitzkrieg (perhaps), Candlemass, Cirith Ungol, Manilla Road et. al. all had solid stuff, but the rock component of their music makes it very hard to praise them duly past their "historical importance." You can see the emergence of the "spirit" that later bands tapped into, but this is not enough sadly.
Off the top of my head Fates Warning (John Arch for best vocalist in Metal?) and Helstar (at least the first half of Nosferatu) is about the only Heavy Metal that I could safely put on a canonical Metal list and feel safe that it fits in with the highest quality Black, Death and Doom albums.