Well, first do you meal literally "girls" as in young girls? I know some parents who buy Valentine's gifts for their children. But assuming the question is asked about a girlfriend: Ultimately what is the best gift is one that shows you know her individual personality. For example, does she actually wear jewelry? If so, jewelry is a safe bet, but not every woman likes it. Same with chocolate if she is watching her weight. I'm sure red roses are always appreciated, but a bouquet of her favorite flower would be better.
I'd say the only rule is absolutely no gift cards for a lover on Valentine's Day! It needs to be a personal gift to be romantic.