Guggenheim Museum Bilbao

Frank Gehry's riverside museum is the building that made cities everywhere want a statement museum of their own. Clad in titanium, limestone and glass, its curves shift color with the weather over the Nervión, so walk the riverbank promenade before you go in. Inside, a tall, light-filled atrium sorts out the three floors, and the largest gallery, 30 meters wide and 130 meters long, holds Richard Serra's steel spirals, which you walk through rather than look at. The rest of the collection is solid but rotating, so the building and the Serra are the reliable reasons to come.