Brunettes! Because they have dark hair and I like dark hair. I don't need to explain myself. Long, wavy dark hair is sexy on a sexy girl. If I find pictures it's going to have more to do with the girls face and her eyes then her hair, the hair just frames that face and those eyes... I can't really objectively look for attractive hair. That's not how it works.
It really depends on the woman's body type. I prefer blond on lighter skin and brunette on darker skin. Blond on dark skin just looks atrocious. Brunette on...well brunette looks good on anything actually. I prefer Brunette.