In descending order I'd have to go with brown, black, red, blonde. With the first three they're pretty interchangeable so hair colour doesn't really factor into whether I find someone physically attractive or not. Except blonde hair, for some reason it's a major turn off for me. Although dirty blonde is strangely one of my favourite hair colours on women.
Also, none of that crazy dyed hair crap. If humans can't grow hair in that colour, I'm not interested. Well maybe if it's monochrome, but that really depends...