There isn't an official stink-o-meter used to gauge the smelliness of a molecule or compound. How bad something smells is a matter of opinion, but most opinions favor the following substances:
Smelliest Simple Molecule
Both of these stinky molecules contain sulfur, which also accounts for the fragrance of rotten eggs and onions. The molecules are detectable at concentrations of ~2 parts per million.
- Ethyl mercaptan (C2H5SH). This man-made molecule is toxic. Inhalation can cause nausea, headaches, lack of coordination, as well as kidney and liver damage. Some people believe it smells like a combination of rotting onion and cabbage, mixed in with a bit of sewer gas. Others think it smells a bit more like old rancid buttered popcorn. This molecule is very volatile and can be smelled in low concentrations, so it is used as a warning odorant for liquid propane gas.
- Butyl seleno-mercaptan (C4H9SeH). This is a natural molecule, produced by skunks. Skunk spray is bad, but modern science has produced odors that are even more vile.
These man-made compounds are more complex and arguably stinkier than the simpler molecules. They also have catchy names.
- "Who-Me?" Five ingredients are used to make this sulfur-based chemical, which smells of rotting carcasses. "Who-Me?" was developed during World War II so that French resistance fighters could humiliate German soldiers by making them stink. In practice, it was very difficult to restrict the application of the chemical to the intended target.
- "US Government Standard Bathroom Malodor" American chemists developed this combination of eight molecules, said to emit a stench resembling that of human feces, to test the effectiveness of air fresheners and deodorizers.