It depends on the girl, but I generally prefer a dark hair. All the girls I've really fallen for had hair that looked black except in direct sunlight. Darker shades of red/auburn are also very nice, but I dislike lighter red/strawberry blonde. I've known some natural blondes that looked batter staying blond than going darker (and a few who looked better going back to a lighter shade of blond that was their natural color when they were younger), but for everyone else I tend to think that the natural shade or darker is best.
p.s. The red head pic isn't showing up, so I can't comment on that one. The brunette you showed is far better than the blonde though.