In small quantities, all three can be nice (in overkill quantities all three are a turn off...). I like silver most, and of course it's important that the girl finds a good overall look with the right clothes, jewels, make-up and hairstyle.
I hate yellow gold so much. It just looks so ugly with almost everything. Rose gold is hard for me to work with, but I think it's pretty. Silver is my favorite out of the choices, but white gold takes the cake as my all time favorite.