Marduk scares the crap out of me. I can't really listen to them. Even as a Christian who listens to some black metal I still cannot listen to them.

Though if we are going into sludgey areas Grinning Mouths by Isis awakens pretty much every intense emotion in my body.

Clearly this award would go to Heroin by The Velvet Underground. Who knew that two chords and a high pitched drone could sound so good.

some ear pulpifyingly intense tunes are:

agoraphobic nosebleed - ceramic godproduct
brutal truth - prey
the dillinger escape place - sugar coated sour
anaal nathrakh - when humanity is cancer

intense but not ear destroying:

today is the day - temple of the morning star (acoustic)
mussorgsky - st john's night on the bare mountain
prokofiev - montagues & capulets
dälek - spiritual healing
neurosis - through silver in blood

Giant Swan by The Blood Brothers.

hair police or wolf eyes easily have anything mentioned in this thread beat.

Good Morning Captain by Slint.

Universal Order of Armageddon- Visible

I'll agree with Me and a Gun - Tori Amos for sure, and add:

Take the veil cerpin taxt - TMV
The Ballad of Robert Moore and Betty Coltrane - Nick Cave (Go listen to that song!)
The Tourist - Radiohead

You Can't Fake the Funk-Trainwreck
My Fecal **** Toy - F*ck... I'm Dead
Anything by Bastardass

I've listened from Agoraphobic Nosebleed to Cryptopsy, from Marduk to Merzbow but still one of the most intense songs I've ever heard is Impulse Manslaughter's Walls.

The build up and the slightly offbeat drums make me frantic.

Play it LOUD!!!

PD: someone mentioned VU - Heroin (good one although intense in a different sense)