Atmospheric black metal.

I know you've heard a lot of good stuff, but I'm always surprised by what you haven't heard, so you've probably already heard some of my recs.

Weakling
Imperium Dekadenz
Wodensthrone
Draugnim
Verganglichkeit
Todtgelichter
Skagos
The Ruins of Beverast
Forest of Fog
Graveland
Melkor
Farsot
 
This is my kind of thread!

Wedard
Njiqahdda
Hypothermia
Nocturnal Depression (the Four Seasons to a Depression EP is very atmospherical)
Ashes (UK)
Forest of Fog
Triste
Fear of Eternity
Totalselfhatred
Old Forest
Nae'blis
Zaratustra
Azuth

And I trust that you're familiar with the obvious Burzum.
 
I'm sort of all over the place with Hate Forest. I reckon that they can be a bit bland and unoriginal at times. Their first four EPs is the way to go imo.
 
Lots of good bands have been mentioned already.

Here are some more recommendation (although I reckon you have heard most of them...)

Forgotten Woods
Austere
Nocternity
Ondskapt
Verdunkeln
Heimdalls Wacht
Lunar Aurora
Lantlos
Geïst
Nocte Obducta
Nagelfar
A Forest of Stars
 
The Ruins of Beverast - Rain Upon The Impure is pretty much my favorite now for this kind of music.
 
Trist (Ger) leans much more to the atmospheric side, but the sixty minute song "Hin" is pretty great. The Willenskraft album is more song-based, but has more of a chunky feel than most BM. Apparently a side project of Lunar Aurora.

Krallice might be considered too busy to be atmospheric BM by everyone, but it's great, and everyone should listen to it anyway. The new album has a lot of slower ambient times, but it's buried 45 minutes into things. Still, great album.
 
Manes (old)
Volahn
Gravecode Nebula (black doom)
Crimson Moon
Nibiru
Nightbringer
Apparitia
Isvind
Xolotl (especially)

Not sure if all of that is atmospheric, but pretty fucking close.

Suprised no one mentioned old Manes stuff. Graveland (despite their racist shit) the Epilog album is fucking very atmospheric, especially the song "Children of the Moon". That song has some of the best keys ever.