Sunn did not impress me. Nott, on the other hand, is pretty brutal.
The new Rings of Saturn video is pretty heavy, but not sure about the heaviest.
Meshuggah does come to mind, especially when considering stuff like the Mayhem Mix of Future Breed Machine.
It may be not the heaviest, but Dissection's song Night's Blood is still pretty damn extreme black metal.
I remember thinking that Carcass's Heartwork was one of the heaviest things ever when I was younger.
And I thought the same when I discovered Morbid Angel.
Napalm Death's Breed to Breathe gets an honorable mention as well.
Nile is kinda brutal.
While stuff like Deicide, Suffocation, and Cannibal Corpse is heavy... I don't think that is the type of brutal you're wanting to find.
How about some Brutal Truth?
Dying Fetus?
Today is the Day is one of the most abrasive bands I commonly spin.
Let's just call it a day and say The Acacia Strain is at least one of the heaviest.
Volumes is a little bit more modern and djenty.
I can't forget about Boris the Blade.
The most pissed off song I've heard is probably Pantera's Suicide Note part II.
It would be really hard, if not impossible to judge the heaviest without some specific measurable criteria. Everything I listed is heavy/brutal/abrasive... At a certain point, they're just damn heavy and damn good at what they do.
If I am really pissed off at the world and want to listen to something heavy Pantera, The Acacia Strain, and Dissection are the first things I would put in my list.