I think Mayhem is a tad overrated. Burzum is the best of the early Norwegian acts, IMO.
I've never delved that much into that area, so I used the responses to an old metal archives thread to decide what to listen to. The thread was about which of the older black metal albums you thought best or some such thing, and I went through the thread and kept track of the responses. The Mayhem album got more attention than anything else by a large margin, so I started there and am working my way down the list, which is...
1) Mayhem - De Mysteriis Dom Sathanas
2) Burzum - Hvis Lyset Tar Oss
3) Emperor - In the Nightside Eclipse
4) Darkthrone - Blaze in the Northern Sky
5) Bathory - Under the Sign of the Black Mark
6) Darkthrone - Transilvania Hunger
7) Burzum - Filosofem
8) Gogoroth - Under the Sign of Hell
9) Emperor - Anthems to the Welkin at Dusk
10) Gorgoroth - Pentagram