I'd say in general Agalloch is the pinnacle of atmospheric BM, but Agalloch also has folk and post (?) in their sound, so they're not exactly typical atmospheric BM like WITTR.
One album per artist in alphabetical order which happen to be my favorites. Feels like an addendum to the ?Beginner's Guide To Building A Prog Collection" (http://www.metalstorm.net/users/list.php?list_id=58) often mentioning the same band but different releases.