The Pompidou Center has to be one of the ugliest buildings in Paris. But it houses some pretty amazing art by Picasso and Matisse and photography by Man Ray just to name a few of the artists represented here. And the museum's observation decks offer some of the most incredible panoramic views of Paris available anywhere in the city. From here you can see almost all the monuments from Sacre Couer, l'Arc de Triomphe, le Tour Eiffel, and Notre Dame to name a few.
For all that, I can stand a little ugly on the outside.