I'm a bit confused as to whether we're discussing great bands or great metal albums. I don't think bands that have released one sole masterpiece should be listed alongside those that have released three or more outstanding albums. At the top i would have Burzum, Darkthrone, Graveland, and Immolation, Gorguts, and Morbid Angel. These are definitely the best that it gets concerning metal bands. Other greats worth mentioning are Beherit, Gorgoroth, Vader, Suffocation, and i'm pleased someone mentioned Fleshcrawl who have released several fucking solid albums during their career.