According to tastessence.com, there are more than 160 varieties of peppers. Amongst them are sweet peppers, which deliver no heat when eaten, such as the common bell pepper. However most varieties are considered spicy ... some to the point of burning one's throat when consumed in high quantities.
Many varieties of hot peppers are used in cooking, and a popular craze today is to sprinkle hot sauce, made with the seeds, rinds and juice of spicy varieties of peppers, onto just about any food. Revved Up™ (RevvedUpSauce.com) is an example of a commercial hot sauce that spans the heat index from mild Jalapeño to throat-tingling Peri-Peri Ghost (blend of Peri-peri, Ghost pepper, and Cayenne peppers). Other popular peppers for hot sauces include Serrano, Habanero, and Carolina Reaper.
The best way to discover which peppers are optimal for your favorite hot sauces is to experiment in your own kitchen.