I don't know about "most attractive" but I'm drawn to red hair because it is the rarest hair color for me to see ever. I haven't seen a natural red-head in years. So for me it's so incredibly "exotic" and I find myself staring inadvertently whenever I run into a very beautiful red-head (extremely rare occurrence).
The colour a woman is born with generally works best. It will always be compatible with the rest of her colourings. The next safest option is to colour the hair something similar to it's natural colour. Something a bit brighter or darker or more varied.
Beyond that, go for your life. The more unnatural options (whether hair colours or skin/hair combinations) tend to work better if the rest of you is particularly good looking, and/or it's a taste a guy has picked up somewhere.
The orange skin/blonde combination doesn't work for me at all (for example), but some people seem to like it.
Depends on skin tone. In general, contrasts work better. So tan skin goes really well with light brown or blonde hair. Pale skin looks beautiful with dark brunette or black hair. Red hair largely depends on if the girl suits the colour and has a pretty face.