I'm not saying these albums are "the best of the best", but I think they are pretty definitive of their genres. I also took into consideration the bands that will be remembered in history.
Definitive to me means "archetypal," or exemplary of what the genre was about, not necessarily a best of or list every album of.
I'd like to do a list for outsiders: how to understand metal music through its definitive acts.
Judas Priest - Painkiller
Iron Maiden - Killers
Mercyful Fate - Don't Break the Oath
Motorhead - On Parole
Metallica - Master of Puppets
Prong - Beg to Differ
DRI - Dealing With It
Cryptic Slaughter - Money Talks
Dead Horse - Horsecore
COC - Six Songs, etc.
Sodom - Persecution Mania
Kreator - Endless Pain
Destruction - fuck, I forget the title
Rigor Mortis - Rigor Mortis
Slayer - Reign in Blood
Pestilence - Malleus Malificarum
Morbid Angel - Altars of Madness
Deicide - Legion
Obituary - Cause of Death
Amorphis - The Karelian Isthmus
Therion - Beyond Sanctorum
Massacra - Final Holocaust
Immortal - Diabolical Full Moon Mysticism
Emperor - In the Nightside Eclipse
Enslaved - Vikinglgr Veldi
Gorgoroth - Antichrist
Burzum - Hvis Lyset Tar Oss