I agree with the comments here. Personally, the sweetest coffee I have tasted is natural processed Kaffa Anderacha, which managed to keep some sort of cherry flavor no matter which roast level. I think you might get a real feel for sweetness in coffee with a quality natural processed coffee from Africa. The only issue here is that it was pretty tart, or sour\acidic, which, to my understanding can be alleviated by having it a little darker roasted, but that might also degrade the cherry taste.