In boxing people say "pound for pound", as in "pound for pound he's the greatest boxer ever." Who is the best metal band ever album for album? This is meant to take creativity, longevity, and consistency into account. Have at!
I disagree with Opeth: Deliverance is terrible and Damnation is boring and Ghost Reveries is boring.
I also disagree with Black Sabbath, as I can't stand Dio.
For me, I'll go with Anathema. They started out as one of the best doom metal bands and ended up as one of the finest rock bands. They aren't all metal though, so for a more pure metal band, I'm going with Converge. Every album is amazing.
For me, I'll go with Anathema. They started out as one of the best doom metal bands and ended up as one of the finest rock bands.
Limbonic Art
Five albums of pure genius, with a sixth on the way.
Primordial
Also, some bands that released less albums: Arghoslent, Mourning Beloveth, Seance, Grand Belial's Key, Moonblood....
There aren't many bands I can think of that have been consistently amazing throughout a long (10+ years) career. It seems most bands either start out poor, end up poor, or both.
My vote would go to Neurosis.