Capsaicin is the chemical found in various hot peppers that causes the lips, the mucous membranes inside one's mouth, the esophagus and each end of the digestive tract to feel a burning sensation upon consumption, digestion and elimination, in addition to being the main ingredient in pepper spray. The hotness of capsaicin is measured in Scoville Heat Units.
Please note: this list should be considered incomplete, as new super-hot pepper cultivars are appearing one at a time every few years, mostly thanks to a man named Ed Currie, one of the world's foremost pepper growers.
Additionally, the peppers listed below have a range of Scoville heat units rather than a static number. This is because there are a few factors that affect the pungency of peppers—weather conditions, mostly, but also soil quality and, as any grower would tell you, luck.
Many, if not most of those listed below following the ghost pepper, are cultivars and are not found in the wild.
||Scoville heat units
|Anaheim (aka New Mexican)||1,500—2,500|
|Hungarian wax pepper||2,500—15,000|
|Chile de Árbol||15,000—30,000|
|Nagabon (Scotch Bonnet X Ghost Pepper)||750,000—900,000|
|Naga Jolokia (aka Ghost Pepper)||855,000—1,041,000|
|Trinidad Scorpion Butch T||1,200,000—1,400,000|
|Genghis Khan's Brain||1,500,000—1,700,000|
|Trinidad Moruga Scorpion||1,800,000—2,000,000|
|Pepper X||3,180,000 +|
For reference, consumer-grade pepper spray rates about 2,000,000 and police-grade rates 4,500,000 Scoville units. Pure, unrefined capsaicin rates at 16,000,000 Scovilles.
It's a tricky proposition to eat any of the peppers (fresh and whole) listed above that come after the habanero; search Youtube for "eating carolina reaper" or "eating scorpion peppers" and it becomes abundantly clear that doing so causes most people severe gastrointestinal distress, making the consumption of such peppers more of a survival challenge than anything else. (Indeed, Dragon's Breath and Pepper X aren't even intended for human consumption; rather, those peppers are grown for use in anesthetic topical solutions, although at least one sauce has been made from Pepper X—Heatonist's "The Last Dab XXX", which has been featured on Hot Ones for a few years.) This isn't necessarily the case when those peppers are used to make hot sauces, but still, use caution and common sense when approaching heat level 10+.