The Smithsonian Museum, in Washington, DC (and elsewhere) is the world’s largest museum. It is actually a collection of numerous museums, all bundled together under the same name. Each building of the Smithsonian would probably rank near the top in its particular specialty, but the total museum outdistances all others in sheer size and in the quantity of its holdings.
The bulk of the museum is located in Washington, DC, on the National Mall, between the US Capitol building and the Washington Monument. The specific buildings making up this group are the following:
The National Museum of African Art
The National Air & Space Museum
The American History Museum
The American Indian Museum
The Freer Gallery of Art & Arthur M Sackler Gallery
The Hirshhorn Museum and Sculpture Garden
The National Museum of Natural History
The National Portrait Gallery
The Smithsonian Institution Building, The Castle
Near the Mall are several other buildings of the Smithsonian:
The American Art Museum
The Renwick Gallery, with one of my favorite pieces of art
The National Postal Museum
The National Zoo
Stephen F Udvar-Hazy Center
where you can see a complete Space Shuttle on display as well as many other huge air and space vehicles.