Seedsavers gives you a heat scale from 0 (sweet) to 5(hot).
I love Golden Marconi and Jimmy Nardello, both of them are prolific and sweet. Poblanos, green, are a chili that is not very hot, just a touch of heat and good flavor. They are also called ancho when red and dried.
Some paprika peppers are mildly warm, most are very sweet and thick walled. Pimiento peppers are thick walled and very sweet when red ripe.
I only plant heirlooms and haven't had experience with hybrids for many years. The advantage of heirlooms is that you can save seed from ripe fruit for next years crop.
If you are buying from the local walmart or mom and pop, sometimes banana peppers are tiny hot and usually rather prolific.
Care Charlie