Gustav Klimt's Portrait of Adele Bloch-Bauer (1907). Ronald Lauder paid 135 million for this painting in 2006. It resides in NYC's Neue Gallery. (Gallery, and the restaurant, are a nice touch of Vienna.)
A pupette and I spent a very pleasant day seeing the Klimts, and other good stuff (Schiele, Kokoshka), at the Belvedere in Vienna two years ago, then moved on by foot, trolley, and subway to lots of other fun things and tasty food treats - Viennese pastries and beers - along the way. She likes Schiele.
Here's the story behind The Mona Lisa of Vienna