Once, as a child, I visited the Louvre in Paris. There was a crowd around the Mona Lisa--a surprisingly small painting--but not around other works.
It's the most famous painting in the Louvre, so it attracts a lot of attention. Writer David Friedman wondered: what do other museums regard at their most famous items on display? He found 17 museums that officials had specifically identified works of art as their own Mona Lisa.
In the case of the middle work above, it's a silkscreen by Andy Warhol of Marilyn Monroe in the possession of the Metropolitan Museum of Art in New York City. Friedman also includes museums of history and paleontology.