A museum I visit quite frequently is the Städel Museum at the Frankfurter Museumsufer – directly opposite to the city center, next to the river Main. The Städelsche Kunstinstitut und Städtische Galerie (correctly named) houses 3.000 paintings, 1.000 typically on exhibition.
The institute dates back to 1815, the building at the current location at Schaumainkai exists since 1878. Exhibitions at the Städel are typically high quality presentations and very informative, giving everybody the chance to see masterpieces of art.
Even the normal collection is worth a visit, starting with Tischbeins Goethe in der römischen Campagna and then rotating through nearly all styles and decades of European art. I favor their section with Eugène Delacroix, Claude Monet, Édouard Manet, Gustave Courbet, Edgar Degas and friends.
Städelsches Kunstinstitut und Städtische Galerie
60596 Frankfurt am Main