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.
Similarly, a shity old shed baking in the sun, filled with garbage bags of fish guts (from guests' fish at the gutting table of a fishing resort), that we had to load onto a pickup truck.
Like you said, a physical wall. I couldn't get past it. Whenever I got near the door of the shed, I doubled over wretching and y couldn't get any closer.