I'd say the following bands are absolute essentials (and their good albums that I've listened to, although not all the albums listed may be essential):
Burzum (Burzum, Aske, Det Som Engang Var, Hvis Lyset Tar Oss, Filosofem)
Emperor (Emperor, In the Nightside Eclipse)
Gorgoroth (Pentagram, Antichrist, Under the Sign of Hell)
Immortal (Diabolical Fullmoon Mysticism, Pure Holocaust)
Enslaved (Hordanes Land, Vikingligr Veldi)
Mayhem (De Mysteriis Dom Sathanas)
Bathory (Blood Fire Death)
Sacramentum (Far Away From the Sun)
Graveland (In the Glare of Burning Churches, The Celtic Winter, Carpathian Wolves, Thousand Swords, Following the Voice of Blood)
Lord Wind (Forgotten Songs, Heralds of Fight, Rites of the Valkyries, Atlantean Monument)
Summoning (Minas Morgul, Dol Guldur, Nightshade Forests, Oath Bound)
Beherit (Drawing Down the Moon, Engram)
Darkthrone (Under a Funeral Moon, Transilvanian Hunger)
Rather than giving other classic albums letter grades, I'd rather simply divide them into an upper and a lower echelon, for example:
Upper Echelon:
Fullmoon (United Aryan Evil)
Veles (The Triumph of Pagan Beliefs, Night on a Bare Mountain, Black Hateful Metal)
Necromantia (Crossing the Fiery Path, Scarlet Evil Witching Black)
Rotting Christ (Thy Mighty Contract, Non Serviam)
Varathron (His Majesty at the Swamp)
Impaled Nazarene (Ugra Karma)
Belial (Wisdom of Darkness)
Branikald (Stormheit, Rdjandalir)
Gehenna (First Spell)
Lower Echelon:
Behemoth (From the Pagan Vastlands, And the Forests Dream Eternally)
Sorcier Des Glaces (Snowland)
Blut Aus Nord (Ultima Thule)
Manes (Under Ein Blodraud Maane) (I haven't listened to this one much yet though, so this is an early impression)
Mysticum (In the Streams of Inferno)
Ancient (Svartalvheim)
Barathrum (Eerie)
Ulver (Bergtatt)
I Shalt Become (Wanderings)
I think a three tiered system for ranking quality black metal should suffice. I would say that all three tiers are worthy of inclusion (although perhaps the DLA could be limited to the first two tiers), with anything bands and albums below third tier status being discarded. I'd say the following bands do not make it:
Marduk, Watain, Dark Funeral, Dark Tranquility
As well as albums like "Belus", "Frost", "Anthems to the Welkin at Dusk" etc.