Also known as Bitrex, Denatonium Benzoate held the record for the most bitter substance on earth until 24th June 2016. A teaspoon of the substance added to an Olympic size swimming pool (volume 2.5M litres) makes the water noticeably bitter. Bitrex has been a very successful additive to poisonous substances to prevent accidental ingestion, such as car antifreeze.
Sadly for manufacturer Macfarlan Smith, since 24th June Bitrex's record has been overtaken by the UK Guardian opinion page. One opening paragraph has the same bitterness impact as approximately 300ml of Denatonium Benzoate. Rumours suggest that Macfarlan Smith has opened negotiations with Jonathan Freedland, Nick Cohen and Polly Toynbee for purchase of their spleens as a manufacturing source of the Bitrex successor.
It is serendipitous that the name "Bitrex" is an anagram of the new product: "Brexit".