Most of these varieties, if not all of them, can be considered as chilli peppers; but they vary greatly in hotness. Wikipedia includes the bell pepper on its Chili pepper page as one of zero hotness.
People who care about such things measure the hotness, or pungency of chili peppers in Scoville heat units (SHUs). The Scoville scale was created in 1912 by an American pharmacist named Wilbur Scoville. The original procedure was to dissolve an exact weight of dried pepper in alcohol to extract the capsaicin (or capsaicinoids – the active components, or what makes them hot), dilute the resulting solution in sugar water, and ask a panel of trained experts if they can detect any hotness in the diluted solution. The SHU value is the number of times the original sample must be diluted before a majority of the panel cannot detect any hotness.
The major drawback of this method is obviously that it's entirely subjective. Different people (even different experts) have different perceptions of hotness, and they won't necessarily rank different varieties in exactly the same order. A more scientific technique uses high–performance liquid chromatography (HPLC) to measure the amount of capsaicinoids in the dried peppers. The results are still quoted in SHUs, but generally as a range of values (to allow for the fact that the same cultivar grown under different conditions can give widely different results).
Wikipedia gives a table of pungency, ranging from "non–pungent" (up to 700 SHUs) to "very highly pungent" (more than 80,000 SHUs). You might like to bear this in mind when perusing the figures below.
There are enough masochists in the world to make the hottest chilli pepper a thing for cultivators to aim for. This is also a popular topic with quiz setters, but it's also a title that keeps changing.
As a kind of benchmark, the jalapeno pepper delivers about 2,500 to 10,000 SHUs (making it moderately pungent) while the Cayenne pepper comes in at 25,000 to 50,000 SHUs (highly pungent).
In 2016, the Daily Telegraph reported that the Trinidad Moruga Scorpion was the world's hottest chilli pepper, delivering no fewer than 1.2 million SHUs. (That's about 200 times as hot as your average jalapeno.) Then in 2018, a new contender came along: Pepper X. This was rated at three million SHUs – two and a half times as hot as the previous title holder (500 or 600 jalapenos).
Other varieties that Wikipedia puts in the 1.5–to–3–million–SHU range are the Carolina Reaper and the Dragon's Breath (the last produced in 2017 in St. Asaph, Denbighshire – in North Wales).
Scariest of all: "most law enforcement pepper spray" is in the same range.
© Haydn Thompson 2020