Musee d'Orsay

One of the best museum or art gallery has to be Musee d'Orsay.  It is famous but many people still do not know about it even when they are from Europe.  (Of course, everyone know about the Louvre)  It is located in Paris, easily reachable by Metro.  It has a large collection of the Impressionist paintings, and also has other styles of paintings including neo classical.  Work by Van Gogh, Monet, Renoir, Pissaro, Cezanne, etc.
The impact of real painting is always more mesmerizing than the textbooks.
The structure of the Museum is also wonderful:

If you ever visit Paris, you have to visit Musee d'Orsay.