Ironically, this museum in Oslo focuses primarily on American contemporary art. Aside from a stellar, innovative collection, the museum is an architectural gem ideally situated on the tip of Aker Brygge. Link.
The next best...
2. Jupiter Artland, Edinburgh
3. Fondation Maeght, Saint-Paule-de-Vence
4. Centro Andaluz de Arte Contemporáneo, Seville
5. Reina Sofia, Madrid