Interesting question. That's one I've been wondering myself. I read from Bonnie Plants' site that some new pepper, called Yummy, is one of the sweetest peppers around and supposedly changes the minds of people who don't normally like peppers. Anyone growing any of those? Is it just marketing BS, and are they really as good and sweet as they claim they are? If I can find some plants around here I might pick one up.
A few others I've been curious about are Gypsy, Cubanelle, Giant Marconi, Bananarama (Burpee), and Sweet Costa Rican (Burpee). Too bad I don't have the space or money to try many this year; I'm going to have a garden mostly full of hot and superhot C. chinenses and a couple jalapeno plants. Oh well... maybe next year I'll try some, as well as several "sweet" chinense varieties (lots of those I wanna try...).
So far, I've only really tried sweet bananas. And hot bananas, but I could barely tell a difference, so either I was doing something wrong or they were just sweet... or there was a mix-up.