The skinniness is definitely the main one. Bah, the men who came up with that just don't have taste. What is the female figure but soft, curving lines to contrast with the flat, square shapes of men? Give me a girl with a little meat on her (Or indeed, a lot of meat on her, even) over an anorexic stick figure anyway.
I don't believe any of them, but I think the most common one is "must be skinny to look good".
However, I will say that most girls look better if they are in shape. And before anyone rages, I'd say the same thing about guys too. Being a little overweight is fine, but once you start going towards obesity, you don't look good. Just... no. If you care so little about yourself (legit medical issues notwithstanding) that you let yourself get THAT fat, then no, you are not beautiful.
Also, since when did large breasts become such a good thing? Skinny chicks with massive tits are gross, imo. It just looks wrong...