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