Despite doing a load of sulfur chemistry in my time (e.g. thiophenol is the pure smell of dog shit) and having smelled putrescine (Putrescine - Wikipedia), they’re not the worst. Too one note.
It was the Tupperware bowl of soup that my housemate made in the first week of us living together, forgot about in the back of the fridge, then opened two years later to get rid of before we moved out. Indescribable.
As a kid I went on a school trip near the end of summer term and found some dead crabs on the beach and me and some other lads decided to take them home as mementos. Got home, unpacked bag of clothes, forgot about dead crabs and chucked bag in the wardrobe for the rest of the summer.
Fast forward 6 weeks, and as if digging out the old school bag wasn’t depressing enough for a new school term, I had to scoop out some mystery goo and shell from the bag and hose it down outside. Nearly threw up so many times.
In our old house, the TV and me put a gammon joint in the slow cooker overnight. There was too much meat, however, and the rest got put back in the cooker to be dealt with later. Completely forgot about it for two weeks until I noticed something was moving under the lid. It was maggots. The small was sourest I’ve ever smelled.