I don't think that heaviness is directly related to "brutality" but I have no idea on how to start defining those things. Anaal Nathrakh, Cattle Decapitation and Nails are certainly extreme, but I don't get the same "heavy" feeling from them than I do from say, Sleep's "Dopesmoker", Wolves in the Throne Room's "Celestial Lineage" or anything by Om.
I've noticed that my perception of time and my ability to focus on things get screwed up when I listen to "heavy" music. I once made the mistake of listening to Dopesmoker while driving - I had to pull over, stop the music and give myself a few minutes to get my head screwed back on right.
It goes like: "Oh, that guitar is quite heavy" and then out of sudden bass comes in. I heard concrete mixers sounding more delicate than bass in Godflesh.
these threads always just turn into a laundry list of bands that the poster likes....
OP asks for your single heaviest band reccomendation. That means single band, not 14 youtube videos of "like this band is, and @2:13 of this track it's pretty brutal, oh and this one other band that my brother plays in - they f'king slay!" /rant