Brown by far. Here's my reasoning: Black: It's alright, although it just lacks texture for me. It's like comparing looking at a computer screen which is showing a flat pink image but then looking at a canvas that's been painted the same shade of flat pink. The canvas has more texture because it's a physical 3d object, therefore meaning light can interact with it. With black hair, it's the same colour as the shadow Blonde: I think it's rare to find a blonde girl which wouldn't look better with a different hair colour. Red: Just looks unnatural, Usually it's obviously dyed.
Brunette without a doubt for me. Out of all the hair colours I have seen, brunettes have been the most consistently beautiful women I have met.
Blondes require a very specific facial arrangement to look good, it's either "meh" or "holy crap" for them.
Redheads can look very attractive, but besides Alyson Hannigan (And her younger near clone Lindsey Stirling) I have never met a redhead where I have stopped and thought "wow, she is absolutely beautiful", plus Carrot Top makes a bad name for them, and the whole "no soul" thing.
Black hair, never cared for. Looks good on men (objectively speaking), but for women black hair just never was a turn on.