True AA flatness is okay but boyish.
Budding A mounds is the peak of perfection.
Modest B breasts are cute, sexy, and feminine.
Generic C cups are okay but a bit boring.
Sizeable D boobs are quite sexy as long as they're perky.
Big E titties is the event horizon between acceptable and unacceptable.
Massive F (and beyond) blobs are solidly in disgusting cowtit territory.