[bradbid]

It seems to me that all the really stinky compounds are toxic or cause irritation.
What are the safe and harmless chemicals with the worst smell?

[Yggdrasil]

tert-butylthiol is used to give natural gas an odor so that people can detect gas leaks.  From wikipedia:

Even in well ventilated areas, extreme caution must be made when handling tert-butylthiol as it is a highly odorous chemical with an odor threshold of <0.33 ppb.[10] Extreme caution is not due to toxicity, but due to the significant odor and the concern this odor could cause to individuals that might be exposed. The PEL for thiols of most types is 500 ppb, primarily due to reaction of nausea at levels of 2–3 ppm. The LC50 of tert-butylthiol is much, much higher.

https://en.wikipedia.org/wiki/Tert-Butylthiol

[Arkcon]

We tested the function of our respirators with Dentonium.  Seems safe enough by injestion, and is very detectable.

[sjb]

Maybe, philosophically, this is by design. These compounds are likely to be harmful so it is worth getting some warning of their existence other than by e.g. direct contact.