I'll narrow it down for you. As has been said, EMPEROR is absolutely mandatory...
My personal favourite album of theirs would be 'In The Nightside Eclipse' but you might also want to check out 'Anthems to the Welkin at Dusk', 'IX Equilibrium' and of course the ALMIGHTY 'Prometheus'.
Having said that, Prometheus doesn't much resemble normal black metal or earlier Emperor for that matter... it seems to transcend black metal and become pseudo-classical. It's pretty hard to listen to and digest, but once you do, you're not goin back.