The really unusual Modern Art museum in Paris is kind of a landmark. It's by far the tallest landmark in the Marias, and it's just so fun to look at.
Inside, the building is home to a few things, including the Modern collection. I am hit-or-miss with stuff like this. I either think it's amazingly creative and visionary, or I have my head tilted with my ear on my shoulder, trying to figure out what it might be.
You also get really good city views from the top as you ride the futuristic tubes of the escalators up to the exhibit space.
The whole collection can be seen in a couple of hours (or, at least at my pace) and it covers a whole range of mediums. It's spacious enough so that even if it is crowded inside, it's never too much jostling, and there's always a lot of art students camped out on the floor sketching.