I worked in a cemetery cutting grass as a teen. Helping out digging a grave one day and the backhoe knicked a neighboring vault. Just a tiny chip smaller than a dime. But suddenly the air over the entire area just filled with the stench of decomposed human remains. It was vile.

Even so, I sure do miss the simplicity of that job sometimes.

For me there are two strong ones competing: Hákarl, basically fermented rotting shark – this smells like a Serbian railway toilet during the war, when it wasn't cleaned for years, but used regularly. It tastes the same. The other was a dog carcass, that was left on the roadside on a nice, hot summer day, and the 'not my job' mentality left it there slowly decomposing over roughly two-three weeks.

My smelling sense went away around 2005, so I'm happy to report this question is not applicable to me. Sadly it took away some percentage of my taste too.

The body odor of a friend who got out of bed when I was crashing at his house. I wanted to throw up on the spot, it was awful. I'm not sure what it was, but it smelled like he hadn't showered in months but he didn't smell like that the previous day.

Dead orca. Came upon marine biologists cutting up and disposing of a beached orca, and the smell was like a physical wall, and then a repeated series of roundhouse kicks to the face and eyes. Just an indescribable stench, regular rotting meat dialed up to dimension-warping, sanity-threatening levels. I will never forget it. I would never ever have got closer, but my girlfriend was driving and she knew one of the biologists and was really interested to chat about it right next to the pickup filled with chunks of tail. Just unreal all around.

Rotten ballistics gel smells a lot like rotting human flesh. So... that.

A dead body that had been sitting in the Sahel desert for a few days. That smell still haunts me.

Hyaluronic Acid

Paper mills like in Ticonderoga, NY or Rumford, ME.

A rabbit necropsy that the vet did while I was on shift. I've been around numerous dog necropsies, but that was my first rabbit.

I almost went home, the smell was so horrific. Smells don't usually make me vomit, but that one did.

Never again, I hope.

Surströmming.

Back in college, I lived in a row of older apartments. Opposite our living room wall was our neighbor’s bedroom. He was found in his bed, 4 days following his passing. Summer. No AC. When his apartment door was opened, the odor spread out and around. It’s a smell I won’t forget.

But the worst smell I’ve had the fortune of smelling is a colostomy bag / baseplate wafer thing.

The restroom of a rural Bulgarian train station. It was just a room with a hole on the ground and little to no ventilation.

While I don't smell it, my girlfriends reaction when driving home from tournaments tells me that the worst smell in the universe is me after a competition...

At a boat show many years ago there was an old dive suit on display. The old canvas type where they had a big circular helmet with port hole windows that could open. Early 1900’s stuff. I took a sniff in it and about passed out. Been over thirty years and I remember it like it was yesterday. Such a putrid and indescribable smell.

There is this remote cabin in the woods belonging to my parents I used to visit frequently, mostly in the summer. I had a chest fridge hooked up to a basic solar panel system I built. One July while visiting I had fully stocked the fridge with food, when I had issues with the solar panel batteries and I couldn’t keep the fridge powered on. I kept putting off cleaning it for the rest of the summer, and then the winter too. Next summer I finally worked up the nerve. Everything, even the cardboard packaging had melted into this brown sludge full of many generations of maggots and flies. The only things semi recognizable was the plastic containers. It took me days and so much bleach to clean it well enough to trust using it again. If it wasn’t such a pain to get out there in the first place I’d have probably tossed the whole thing, but I’m cheap. Still have it, though I don’t get out there much anymore. If my battery dies again I’m throwing everything out right away!

My family stopped at a Dairy Queen and then went on a joy ride. We got stuck behind a truck hauling a tank of liquid poop. The cap was off and it was sloshing out of the hole. Worst ice cream ever.

Pyridine is pretty horrible... Don't really know how to describe the smell: like a mix of rotten cum and ashtray? It overpowers any other smells, gives me instant headaches and the stench can linger for hours.

Dimethyl sulfide is also pretty disgusting to me, kinda like... a kitchen that's been used to cook brussel sprouts for 20 years + garlic breath?

EDTA (https://en.m.wikipedia.org/wiki/Ethylenediaminetetraacetic_acid) smells like concentrated acetic acid mixed with concentrated ammonia.

It is insane

Raccoon pee used for hunting and scented artificial worms used for fishing.

Just yesterday I was cleaning up dog diarrhea and gagged about three times in the process. Had to take breaks and go outside to breath halfway through. Poor girl is really sick though.

Pet boa constrictor died and started fermenting in the viv. It was like a wall of putrid stench. Had to get her quadrouple bagged and taken to the vets for proper disposal.

Rotten salmon in my fridge after making Sushi 4 days before. I've never went from hot to cold to hot just from smelling before.

This is harmless in comparison to what has already been mentioned, but vomit tempts me to add to the source of the smell.

Some babies have some extra wholesome smelling diapers too.

A client that apparently didn't took a bath for a long long time. The person's smelled like piss, sweat and dirt. It was horrible. I don't know how I survived the encounter because the person wanted to talk forever. God bless I'm far away from that position now.

My dog's breath after she gets into the compost pile.