The Wallraf-Richartz-Museum is the oldest museum in Cologne and houses one of Germany's finest collections of European painting, spanning from the medieval Cologne School of the 14th century through to French Impressionism. The medieval German masters are a particular strength — there are few better collections of Gothic panel painting in the world — but the later rooms hold Rembrandt, Rubens, and a strong group of Impressionist works including pieces by Monet, Renoir, and Cézanne.
The building, opened in 2001, is a severe modern structure that fits rather awkwardly with the old town surrounding it, but the galleries inside are well lit and the collection is displayed with intelligence. Plan two hours minimum; longer if the medieval rooms capture you.