I've found girls with every hair color attractive, even radical colors like cun7n1nja's bright orange or blue hair. But red is definitely my favorite, followed by jet black; those will get my attention the fastest.
My last Girlfriend was blond, though, and the one before that was brown, so hair color is definitely not a huge priority for me.
What hair color looks good on you completely depends on your features, skin tone, etc. But if all of those were ideal for the hair type, this would be my list:
Abnormal coloring is very particular. It can look good or horrible. If it is pulled off correctly I think it can easily be number one on my list. I think it can look pretty hot. think: Ledamonsterbunny