Album for Album

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.
 
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.

Kill yourself. Srsly.
 
I was going to say Darkthrone a while ago, but I figured it would sound too typical. But still, my answer would probably be either Darkthrone, Countess, or even Bolt Thrower. Yes, Bolt Thrower.

edit-Emperor doesn't quite make it. Almost, though.
 
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.
 
Candlemass, Black Sabbath, Countess, and Graveland are a few other notables with an extended career/discography that have generally maintained the quality more or less.