I'll expand further on where Memnoch started. I feel that just taking one from each of those bands is actually just too superficial considering the role each of those bands played in the solidification of the sound of the genre (not that he implied differently).
When it comes to Mayhem, I definitely have folks start with "De Mysteriis...," then I have them look elsewhere for a while before returning to the more raw works of Mayhem. "Live in Leipzieg" is completely essential, but its going most likely take a more seasoned ear to be able to appreciate it properly. There is other Mayhem definitely worth checking out, but not so much in the "primer" stage. I will note that "Ordo Ad Chao" is one of the most fucked up releases to repeatedly grace my ears.
Emperor. Every album pretty much signified a new stage for Emperor eventually transcending Black Metal with just a complete mesh of Extreme Metal influence. "Anthems..." is most often regarded as the pinnacle of their work, but I'm not in that camp. I'm not in the somewhat smaller, but little by no means, group that says "In the Nightside Eclipse" is the pinnacle either. Both albums are absolutely essential in my mind, and are absolutely pinnacles of their styles. However, I can't help but be massively in love with the "Emperor" EP. I just think that the versions of the songs on that EP, which is also on an essential/legendary split with Enslaved, are the best recordings. However, that and especially "Wrath of the Tyrant" are ones I had to eventually come back to with a more seasoned ear in order to truly grasp their greatness. Later Emperor is good, "IX Equilibrium" was my first, but again I find they have little do in the scope of a Black Metal primer.
With Darkthrone, again we have various eras. From Stockholm-style Death, to the minimalistic Celtic Frost worship that made them legendary, back and forth from Death & Black and then to Crust this band has been all over the place. I'm just going to say for Black Metal primer purposes "A Blaze in the Northern Sky," "Under a Funeral Moon," and "Transilvanian Hunger" are essential.
Enslaved is another early True Black Metal band that eventually transcended the genre. Obviously from the beginning they were taking a more Folk oriented approach. I would have to highly recommend "Vikingligr Veldi" for a primer, and then expand to Eld. "Hordanes Land" from the split with Emperor is some of my favorite material from the band, but again its one that may take a little more of a seasoned ear to appreciate. From there you pretty much enter their Progressive Extreme Metal era. I think the pinnacle of their post 1998 work is "Mardraum: Beyond the Within." I appreciate everything this band has done, even the very much loathed Progressive Rock oriented "Vertebrae."
It always amuses me that Burzum is somewhat of a taboo band, and even a controversial decision to include in essential Black Metal conversation. I think that is ludicrous. "Hvis Lyset Tar Oss" and "Filosofem" are essential, and if you don't think so I think you're cracked
Only one album, but absolutely essential and pretty far out there is Ved Buens Ende's "Written in Waters." Early Black Metal experimentation that absolutely worked.
A lot of people like to call Judas Iscariot a Darkthrone cover band, but I've always found that JI took that style to places that Darkthrone did not. Of course, there is also the fact that Andrew Harris was doing Judas Iscariot at the same time Darkthrone was releasing their earliest seminal Black Metal albums. JI took a back seat to Sarophagus for years, and didn't really see full lengths hitting the streets until the genre had solidified its base sound. That being said "Heaven in Flames" and "To Embrace Corpses Bleeding" are two albums that have gotten more spins out of me in the last 10 years than just about any other album (another would be Clandestine Blaze's "Fist of the Northern Destroyer). The latter of the two is most likely hands down my most spun album ever.
I'm kind of curious which Anaal Nathrakh you are referring to since they've been playing a style of Black/Death for the last half decade. Their early aggressive Black Metal stuff is decent, but I do enjoy the route they took.