The hotness of a pepper is measured in Scoville Heat Units (SHU). [Named after Wilbur L. Scoville, a pharmacologist for Parke-Davis Company, who first tried measuring a pepper's heat. It was a highly unscientific method of measurement, having testers taste the capsaicin oil extract from the peppers diluted with sweetened water, measuring how much water it took before they could no longer detect heat. For example, a jalapeno rated at 2,500 SHU would be 2,500 parts of sugar water to one part Jalapeno extract. The higher the SHU, the hotter the pepper.
Today SHU are measured with a liquid chromatography machine. It's more accurate, but as with all vegetables, vegetables will vary with their growing conditions. Soil, fertility, temperature and other growing conditions will all affect the heat of peppers.
Naga Viper Pepper, measured at up to 1,359,000 SHU, is currently billed as the hottest pepper in the world. Naga Viper Pepper was bred by Gerald Fowler, who runs the Chili Pepper Company in Cumbria, England. However I have not yet found a source for these seeds.
The runner up, at an impressive1,041,427 SHU, is the Bhut Jolokia Pepper, also known as Naga Jolokia, Ghost Pepper, and the California Death Pepper and is generally considered to be the hottest pepper in the world, because few people have tried growing or eating Naga Viper pepper. Bhut Jolokia bumped Red Savina Habanero Pepper, which came in at a mere 580,000 SHU. The next 3 were: Habanero Pepper (350,000 SHU), Datil Pepper (300,000 SHU) and African Birdseye Pepper (175,000 SHU).
As a comparison of peppers:
- Sweet Bell peppers: 0 SHU
- Jalapeno pepper: 2,500 SHU.
- Cayenne pepper: 30,000 SHU
- Habanero - 350,000 SHU
- Naga Viper Pepper: 1,359,000