Let's come up with a list. These should be metal bands. No need for proto-metal, prog, hardcore, or other bands that were influential but not metal (so no Zep, Floyd, Rush, Discharge, etc.). These bands should be innovators and extremely influential. If they were neither of these, then their work should be among the best the genre has ever seen. Popularity per se does not equal importance, but it goes without saying that popular bands are inherently more influential. I propose the following bands which should be unquestionable locks for this list:
Black Sabbath
Iron Maiden
Judas Priest
Metallica
Slayer
Bathory
Carcass
Mayhem
Darkthrone
Burzum
Motorhead
These bands should all belong, but I could imagine some disagreement:
Death
Possessed
Morbid Angel
Suffocation
Emperor
Mercyful Fate
Candlemass
Venom
Opeth
Dream Theater
Maybe:
My Dying Bride
Celtic Frost
Megadeth
Atheist
diSEMBOWELMENT
Evoken
Saint Vitus
Pentagram
Sodom
Massacra (Death/Thrash)
Artillery
Sabbat (UK) and (Jap)
Heathen
Tormentor
Master's Hammer
Root
In Flames
At the Gates
Cynic
Enslaved
Godflesh
Summoning
Napalm Death
Repulsion
Overcast
Dismember
Entombed
Black Sabbath
Iron Maiden
Judas Priest
Metallica
Slayer
Bathory
Carcass
Mayhem
Darkthrone
Burzum
Motorhead
These bands should all belong, but I could imagine some disagreement:
Death
Possessed
Morbid Angel
Suffocation
Emperor
Mercyful Fate
Candlemass
Venom
Opeth
Dream Theater
Maybe:
My Dying Bride
Celtic Frost
Megadeth
Atheist
diSEMBOWELMENT
Evoken
Saint Vitus
Pentagram
Sodom
Massacra (Death/Thrash)
Artillery
Sabbat (UK) and (Jap)
Heathen
Tormentor
Master's Hammer
Root
In Flames
At the Gates
Cynic
Enslaved
Godflesh
Summoning
Napalm Death
Repulsion
Overcast
Dismember
Entombed