The Museo Nacional de Bellas Artes (Fine Arts Museum) of Argentina located in Buenos Aires holds the most important collection of art in South America. Located in the neighborhood of Recoleta it has really easy access from the Recoleta Cemetery and most hotels there.
The entrance is free and the collection is composed by reknown international artists like Rembrand, Manet, Goya, Van Gogh, Degas and more; and the best argentine painters from the 19th to the 21st century. The museum is not overwhelming and has the perfect size to enjoy it without getting exhausted by so much art or too much of the same style. A visit is really recommended.
If you are really fond of museums after a visit you can walk just 2 blocks to take a look at the stunning palace of the Museo Nacional de Arte Decorativo (National Decorative Arts Museum)
If you want to know about more museums in Buenos Aires check this post!