3. Metropolitan Museum of Art New York, N.Y.
www.metmuseum.org
Craning your neck at the Met's enormous façade on Fifth Avenue, you might guess this museum is impossibly huge. You don't know the half of it. This is one seriously comprehensive art museum that doesn't bother with petty period constraints; If an object was created between the Stone Age and now, it may well be in here. The Met is the preeminent American museum of art, and its collection of European works is matched only by that of the Louvre.
If you're trying to decide how much time to spend, know that the museum holds over two million pieces of art. At a rate of a minute per piece, you'll need roughly four solid years of viewing time, so you had best get started.