If you're looking to get some more BM, maybe try Emperor's In the Nightside Eclipse. An absolute masterpiece.
Aside from that one, some BM must haves are, IMO
Mayhem, De Mysteriis Dom Sathanas
Burzum, Hvis Lyset Tar Oss
Darkthrone, A Blaze in the Northern Skies
Immortal, Pure Holocaust
Also, if you can ever find it, Graveland's Carpathian Wolves album is quite incredible.