Heaviest song you ever heard?

I think "heaviest" is subjective -- everyone can easily have different definitions of "heavy" music, even for just what would "send headphones flying". Additionally, there is no single heaviest than there are several.

Any Nile track I've heard sounds quite heavy and dark. Also, Cannibal Corpse in general. Seems so with many death metal musicians. If black metal, I'd say early works of 1349 and Darkthrone's "Det Svartner Nå", for instances...

Slayer - Jesus Saves

Honourable mention to Bathory - Born for Burning

Anything by machine head

⚠️ Bring a pair of scissors before listening - If the headphones don't fly, but rather begin to squeeze your skull, throw them away, immediately! (if they don't come away, cut the jack with the pair of scissors)

Lingua Ignota - Do you doubt me traitor 

Flagitious Idiosyncrasy in the Dilapidation - Visualized

Endon - Through the mirror

Carcass - Ruptured in Purulence

Pantera, Cowboys From Hell.

Slayer, Reign In Blood

Motorhead, Orgasmatron

Judas Priest, Metal Gods (live)

Bauhaus - Double Dare

Hey now guest.

This one would be the junior among the answers, but I like it for the texture...

Deafheaven - Dream House

Cheers man.

What about "Hurt", originally by NiN but it's the Johnny Cash version that really does it for me.

Korn - 4u

Hard in the sense that it's deep for me. ;-) My lifestory.

Edit: Yeah, Idgaf. I answered honestly. 😎 What you're looking for comes a dime a dozen.