Hidden on a back rampart of the vital nonprofit Southbank Centre, it hosts terrific blockbuster shows, which have included Ansel Adams, Roy Lichtenstein, 1920s Surrealism, and a 60-artist panorama of modern African art. Always modern, never staid, and a recent multi-year renovation put it back on London's art A List—make an advance booking. It also takes itself quite seriously, which can be good for a laugh