I think that any doom metal can't be described by the word 'extreme'. To me, at least, extreme is usually incredibly technical, incredibly fast, and incredibly heavy. Sad, depressive music doesn't usually lend itself to speed and heaviness. Don't get me wrong, I enjoy sad stuff as much as I do the brutal stuff, and the black metal genre is definitely full of it, but the most emotionally-stirring sad music is going to be slow and melodic, and not 'extreme'.
I'm not trying to shit on anyone's tastes here, but you can give me a black metal song and then show me a song like "Limbs" by Agalloch or At the Gates of Sleep by Insomnium and I think it'd be hard to beat songs like that in terms of being full of emotions such as sorrow and anguish and all that good stuff.
