While it is nice to listen to your daughter's feelings, in order to create a healthy world for her, you must encourage her to see her dad. If she respects your opinions, use that to persuade her to work on her relationship with her dad. Do you and dad get along? Maybe let him know how she feels. Chances are he hasnt payed close enough attention to know that he is hurting someone that he loves this much. Even if he does have a favorite, he still loves her. And, if you feel that there is favoritism, do you openly support her opinions? Even if she hates him, it will eat her up inside on a daily basis until she learns to let go. Helping teach her this will make you a better parent and build a stronger bond between the two of you.
I think she has to be 16, but it may depend on your state. I think she should try to work on her relationship with her father.
This is coming from someone who was estranged from her father from the ages of 15 to 24.
We have an excellent relationship now.