Man, that fucking opening of Book of the Fallen still gives me chills. Totally a capella and perfect harmony.
That's the only a capella moment of Caladan Brood that's been recorded as of yet, but I would add to it the wonder that is Wild Autumn Wind. About 9 and a half minutes in, it takes a step up from great to amazing, and the vocals after the solo are absolutely breathtaking. I'd actually consider them better than those in Book of the Fallen. The ending chorus is also wonderful.
I'm going to piggyback on your post a bit because I feel like there are things you completely missed.
Opeth is a band I've had a long bizarre relationship with, from love to apathy, but in the early to mid 2000s they could do no wrong. And Mikael certainly has a hell of a voice.
And I love me some Bjork but what always stuck with me was her older stuff, this in particular:
She sounds like she has a throat full of sand through the whole song and it still manages to sound amazing.