I've seen a lot of really shitty "essential metal albums" lists so I made one of my own. An attempt to cover all sub-genres thoroughly was made. I've defined essential based on this criteria:
* Effect on forming genre/sub-genre in terms of sound
* Influence on other bands
* Recency is taken into account negatively
* No "proto-metal"
I tried to include no more than 2 albums per band and only more than one if the band had different distinct sounds. Of course Black Sabbath gets a clear pass on this one. Albums start with Black Sabbath at 1970 and end at the year 2000.