Gah that's tough.
1. Burzum- First five black metal records are all classics. Even the weaker records are solid.
2. Deathspell Omega- Everything from 04 onwards has been amazing
3. Emperor- Three absolute classics (ITNSE, ATWAD, Prometheus)
4. Darkthrone- Four classics (Blaze through Panzerfaust) but a lot of so-so records in their middle phase.
5. Agalloch- This is the tricky one... they're a lot more than just black metal, but judged on the whole, they're closer to black metal than any other genre. First four records are all classics.
6. Graveland- Everything this guy released between The Celtic Winter and Memory and Destiny was very good to great. The later part of his career has been quite predictable and pedestrian.
7. Enslaved- Just extremely consistent. VV is the only record I'd consider a straight-up masterpiece, but on the other hand, every single album is good if not very good.
Now it gets really hard, because now it becomes a choice between bands that released a few great albums and bands that released many good albums...
8. Mutiilation- This is based almost exclusively on Remains and Vampires. Two of the best black metal records ever. The later stuff is pretty mediocre.
9. Nagelfar- Three epic full-lengths
10. Immortal- First two albums are classics, next two albums are solid, but the final three records are so mediocre. Still they did just enough to get the nod.
Honorable mention to Velvet Cacoon. Genevieve is one of my top 10 black metal albums. I'd say they just don't have the quantity of records as a band to make the list.