What might be called the best hair length depends on the individual female. Natalie Portman is absolutely beautiful with short hair, very short hair Heck, she's beautiful when her head is shaved. That just won't work for some women, some need some real length to help bring out or enhance their looks.
In the late 70's I lived with a girl who had hair about half way between the bottom of her 'bottom' and her knees. It looked great on her, but it really got in the way. I was always kneeling on it or something. At times it seemed to be everywhere. But it sure could look sexy spread out over pillows and a bed.