Red pepper flakes might be an unexpected answer to What is a substitute for paprika?, but trust me when I tell you that it's one of the best paprika substitutes!
Food scientists classify cayenne as a medium-hot variety of pepper. According to Colorado State University Extension, this spice clocks in at between 30,000 and 50,000 units on the Scoville scale, a method of measuring the amount of capsaicin in a hot pepper.