There's no single right answer, of course, but lately I've gone in big for the various yellow ajis: aji limon, aji pineapple, Guyana, aji amarillo. None of them are all that hot, but their flavor works and plays well with my palate, and they deliver just the right amount of heat for a lot of "don't kill the dinner guests" uses. I recognize the soapy flavor but it doesn't bother me, and in a strong-flavored sauce context like a ceviche it mostly disappears.
On the other hand, I think the Fatalii is perfection incarnate in the light-heavyweight class. You'd never use it for the same things as an aji limon though! The trick is to find a balance where the flavors of the dish are strong enough to compete with the heat, yet don't blow away the citrus/floral character of the pepper. I like it in chicken or shrimp vindaloo, where the habanero flavor would clash with the other spices IMHO.
-NT