The hottest chilli is Smokin Ed's 'Carolina Reaper', grown by The PuckerButt Pepper Company (USA), which rates at an average of 1,569,300 Scoville Heat Units (SHU), according to tests conducted by Winthrop University in South Carolina, USA, throughout 2012.
The Scoville Unit (SHU) scale is a method of quantifying a substance's 'spiciness', through determining the concentration of the chemical compounds responsible for the sensation, which are named capsaicinoids.
We have American chemist Wilbur Scoville to thank for the scale that rates the chillies we chomp. In 1912, long before high-pressure liquid chromatography tests in labs, he relied on taste alone. A grain of chilli was dissolved in an alcoholic solution added to sweetened water until it could barely be noted by a panel of testers. The more dilution required, the higher the rating.
Chilli Scoville rating
Cayenne 30,000–50,000 Tabasco
Scotch bonnet 80,000–300,000
Bhut Jolokia 800,000–1.04 million
Naga viper 1.38 million
Carolina reaper 1.56 million (the hottest chilli)