I'm going to go with piano. I don't know if this is 100% true, but I think that if you master the piano, it's easier to learn other instruments.
I knew a girl in college who knew piano, taught herself how to play acoustic, and ended up becoming a Youtube star. Got to be a guest on "Ellen" and everything. In college, I asked her how she figured the guitar out and she said it was b/c she knew the piano.