Depends on her skin tone I think. For example, it looks really silly when an Asian girl goes blonde but looks good if she chooses red. White girls look strange if they dye their hair black especially if they have really pale skin. Other than that I would say I lean towards blond for most :)
I have a thing fro brunettes with naturally straight or wavy hair... but not just based on their hair color but the physical traits and characteristics that brunettes with that hair color tend to have.
Honestly you cannot answer this because LOTS of girls are pretty and LOTS of girls have differnet hair colors. Most girls are beautiful with their natural born hair color or dyed hair... lol so I can't say that Blonde is the most beautiful because some girls would NOT look good blonde and some would look pretty with blonde hair... you know like some girls CANNOT pull of black hair.. but some most definitely can... therefore there is no set beauty standard for hair.. Most girls are pretty with the hair color that works best for them