I like a combination of the two. Not too curly, but not too straight.
Curls that go in every odd direction usually do not look good, and girls with this type of hair generally keep their hair short to keep it under control.
Straight hair that has absolutely no curve to it does not look good either. Some girls have such straight hair that it looks like individual strands are simply glued to their head. This is not attractive either.
The best is when a girl's hair has some wave to it. This is a combination of curl and straightness. That looks best.