With Emperor, which is the only Black Metal band I listen to regularly, you probably are best off strating with In the Nightside Eclipse. Even thoug I think Anthems to the Welin at Dusk is superior, in retrospect I feel i should have started on Nightside.
It's a lot closer to how they first started off, if you can get through Nightside and Anthems and enjoy them, because they're awesome and are really Emperor's golden era, then go back to the earliest stuff.
I believe Wrath of the Tyrants conatins the most recorded stuff from their early era, if you listen to the likes of Lord of Storms and Wrath of the Tyrant, then you'll really come to appreciate just how much the matured between then and their golden era, it's not easy listening.
After that, continue in order probably, they went a bit more progressive on Equilibrium and frankly mental (in comparison to the early stuff) on Prometheus, alright stuff though.
If you want to get totally into them Reverence is worth buying, but it's not a must have release. The Imperial Live Ceremony is pretty fun though, from a fans perspective.
What else is in your chain, O lord of sound?
