MAC eyeshadow of your choice :) good luck with your contest!
I agree with lovesparks.xo. A Mac eyeshadow would probably be the best choice, because some people don't wear nail polish or lip stuff as often as they probably use the eyeshadow.
I'm all about nail polish, so the O.P.I. would be my pick. They are all great prizes though!
I'm a little late. (:But MAC eyeshadow of your choice!