It depends a lot on what you are able to tolerate. For people not use to eating cayenne the "cool cayenne" is a better choice. This generally runs around 35,000-40,000 heat units (HUs). For people use to cayenne but cannot tolerate really hot peppers then there is the standard cayenne, which runs around 90,000-100,000 HUs. If you can tolerate the really hot cayenne ("bird pepper cayenne")then this will do the best job. But it is definitely hot averaging 160,000-180,000 HUs. You can also find these in the nurseries sometimes sold as ornamental peppers. They are small conical red peppers, and sometimes long and thin, growing upward instead of down. I like putting them in my chili, which makes me just pour sweat.
You do not need much cayenne to work. A quarter of a teaspoon sprinkled on your food should work fine. There are also capsules for those who do not want the burning mouth.