Oh this is hard. I think I would write to Hillary Clinton to ask about her marriage, her regrets, whether female leadership is different than male leadership, and what keeps her motivated. I think she is interesting and has made a lot of sacrifices in her career, so I'd like to get a sense of how to find a good balance and what's worth sacrificing.
Have always wanted to write Audrey Hepburn and tell her that her character inspires greatness in women of all ages :) I would tell her that my 9 year old daughter adores her for her unique beauty and view of the world. To tell her that Breakfast at Tiffany's, since I was young, has been my beautiful escape! And thank her for being an inspiration and a beautiful soul.
I would write to myself when i was about 14 years old. I was so doubtful of who I would be and how I would turn out... I just feel like it would put me at ease after I knew I had gone to college, found the love of my life, and ended up living a sometimes full life. (There is more to come of course.)