The Louvre may be Paris' most famous museum, but the Musee d'Orsay holds many of the most famous paintings and is my favorite of the two. This museum picks up where the Louvre left off and focuses on impressionism, especially on the top floor, which includes Monet, Manet, Renoir, Cezanne, Degas, etc. Here you will find Degas' ballerinas and Toulouse-Latrec's posters, as well as sculptures and architecture.
It closes one day a week and lines can be longer than those at the Louvre, so get there early. It can be a little confusing getting from one floor to another. The first time I visited, security evacuated the building, but the whole thing only took about 20 minutes.