I've always been partial to long (just upper back, shoulder-blade-length or so) darker hair.
As many other people say, though, it's completely dependant on the person, and personal tastes. I've seen some short haircuts on girls that make them look absolutely stunning, and I've seen some long hair cuts on girls that simply make me go "meh".
Worth noting, though, is that I've let my hair grow down to my butt, and got mistaken for a girl pretty often. I had my hair cut and styled a few months back, but haven't had the chance to go back to the hair dresser's so it's about shoulder-length now. I get mistaken for a girl sometimes when it's like this, and if I put it in a pony tail I know it looks girly (long bangs that curve inwards just under my chin). When my hair was just cut, and still quite short, I was never mistaken for a girl.