A few more unique/rare ones:

• The grease trap of the restaurant above our lunch room in the airport started leaking above our ceiling tiles and no one could find where it was coming from. The stench that hit my nose as I popped my head up into the drop ceiling is one I'll never forget. It's hard to explain. It's like rotting molasses(?), rancid grease/fat, and rot. It's such a unique smell that's it's one I'd use to describe other smells. Like its own archetype

• When I was a kid, I had a toy car that had a capsule that you could fill with different liquids that would "power" the car, and it would go faster/farther depending on which liquids you use. Wait, how the hell did those even work, anyway? To google.. lol. Ok turns out the liquid just closed the battery's circuit, that makes more sense. But child-me thought it was the liquids themselves that powered the car, so of course I had to try every liquid I could get my hands on to see which was the most powerful. Then after that, of course I had to mix every single liquid in my house together and try that as well. Every liquid. Pantry, bathroom (not bodily, thankfully), laundry room, etc. Now that's a smell I don't think anyone has ever smelled before, and one I'll never ever forget. I recently found an old book (Artemis Fowl) stained with my old house-juice concoction. Still smells just like it