I think my preferred hair type is "long and clean" but if we're restricting ourselves to colours, it's black, then black, then dark red-brown, then very dark brown, then brown, then probably really deep gold. My least favourite has to be purple. I don't know what weird cult requires purple hair of all its initiates, but I grew sick of being surrounded by its members all sporting its colours like they're something special.
As many others have said, though, it's not the look of the hair that's really important. It's the person living in the head that grew it.
Of course, if you're looking through a collection of NSFW sample galleries [http://shuqoyqa.livejournal.com/] from some "art" site you don't get so much of the personality ... and there I'd say my favourites are black, black, black and red.