They were great indeed , not that their instrumentals and all were that amazing or groundbreaking but the subject matter and the atmosphere they brought was and still is pretty insane !
I'm surprised that I see Slipknot is getting mentioned in most lists since they catch a lot of hate by the so called Metal purists....
Metallica (Ride - Black) Best Metal Ever! Pantera (Cowboys - Steel)
System of A Down (System - Steal)
Rob Zombie/White Zombie (Soul - Hellbilly)
Acid Bath (Kite and Paegan)
Tool and Faith No More are my favorite bands but I don't classify either as pure metal.