The Upper Belvedere houses the Belvedere Museum, where Austrian art mixes with grand state rooms. Like the Louvre, you’re liable to run into a mass of people standing around one famous painting – in this case, Gustav Klimt’s The Kiss.
To tell you true, I think you’ll find better art (and fewer people) in the Schloss Eggenberg in Graz. However, there is plenty to see beyond Klimt, including portraits by Thomas Lawrence & François Gérard and work by Klimt’s contemporary, Egon Schiele. If you’re tired of oils, don’t miss the contorted “character heads” carved by the 18th century sculptor Franz Xaver Messerschmidt.