I'ma have to agree with dbbc on this one and go with Buster Posey, with Yadier Molina 2nd and Kurt Suzuki 3rd.
What makes Posey better than Molina? The way Molina has improved every year I doubt if you can even make a strong argument that Posey is better in the batters box, and Molina is probably the best ever on defense which is what were talking about anyway!