depends on the girl, her natural hair, hair length and style, and overall look.
some girls have beautiful natural hair. others look better with styled hair. it also depends on the length and cut, certain haircuts look bad if they aren't styled properly, especially mid length that hit around the shoulder, I find that longer hair is more likely to look better natural.
i think most girls, regardless of their natural, look good in straight hair that hits about mid-back length.
personally I love a girl with long curly/wavy hair but as long as she's happy with it and it's not shorter than shoulder length than it's fine. (very rarely short hair can look good but generally it does not)