Their BM or DM stuff... or both? I haven't heard their early stuff, but I was quite impressed by Demigod.
I agree - Once Was Not is good.
I'm not sure if this is unpopular, but I prefer Dark Angel - Time Does Not Heal to Darkness DescenRAB. Also, I think that Windir makes some of the most beautiful and atmospheric music I've heard, and that's coming from someone who listens to a lot of post rock/metal.