Most guys prefer brunettes most of the time. Long story short, this is because on a percentage basis, brunettes are prettier than blondes. That is, it's much more likely for a brunette to be prettier than a blonde. That said, there are many smokin' blondes who will completely blow away pretty brunettes. There just aren't as many of them.
At the end of the day though, it's all subjective. Guys look at the total package - body, face, teeth, clothing, personality, etc. and how these features come together as a whole. Hair color is a much smaller factor than many women think.
I have a slight preference for blondes, but I live by the adage that "hot knows no color." I like both.
I'm a guy and I know these things.