As something that can be moved... it would be Pieter Brueghel the Elder's Wedding Dance.
http://en.wikipedia.org/wiki/File:The_Wedding_Dance.JPG
Only about 45 confirmed Brueghel's are known to exist worldwide, with 1/3 of them tied up in Vienna's Art Museum. Most of them are in European museums, with probably fewer than a 1/2 dozen elsewhere. Not only is it valuable because it's rare, but Brueghel painted 2 major scene types... landscapes... and peasant genre scenes of common folk in everyday life. This is of the 2nd type... and among those it's one of the very best, with over 100 people in the scene. He painted several peasant wedding scenes, and this is his finest. It's pained not on canvas, but on wood.
Another very valuable artwork is the Diego Rivera Murals... "Detroit Industry". Many art critics say that Rivera, the greatest Latin American painter of the 20th century, created his finest work here in Detroit, and at Rockefeller Center in NYC. But of course the Rockefeller's hated the communist connection that Rivera painted into their mural, and within a year had it destroyed for a much tamer [[and lamer) mural by another artist. So the surviving Detroit mural wins in that department.
Of the American paintings... Frederick Church [[late 19th century "Hudson River School landscape painting")... his work "Cotopaxi" is my personal favorite, and arguably this mammoth painting is one of the DIA's crown jewels.
http://www.dia.org/object-info/baeac490-f496-4a17-b917-dd0216d11492.aspx
Church, and his Hudson School cohort Albert Bierstadt painted huge canvasses of the American west. These massive canvasses traveled around the country and to Europe to show the magnificent beauty of the American West. However Cotopaxi is a painting of a volcano in Ecuador [[tallest active volcano in the world).... and the colors are outstanding. Both Church and Bierstadt paintings have seen enormous gains in both popularity and value over the years. I believe that Cotopaxi was a steal at $700,000 back in the 70s.
On a different note... DIA Founders Society Chairman Richard Manoogian owns George Caleb Bingham's "The Jolly Flatboatmen".... the best of a series that he painted about life on the Mississippi in the early 19th century. Many art critics call this painting one of the 1/2 dozen most important American paintings of the early 19th century.
http://en.wikipedia.org/wiki/File:George_Caleb_Bingham_The_Jolly_Flatboarmen.jp g
When Manoogian bought the painting many years ago [[I think he paid $5.5 million), it was on loan to the National Gallery in Washington on the Mall, with the stipulation that it stay there for a set period of time longer. It recently went on tour, and its' current whereabouts I have not been able to ascertain online. Perhaps Manoogian's picture will be coming home to Michigan [[he has a collection of art at his Masco HQ in Taylor, and a lot of masterpieces on loan or gifts to the DIA). So technically this painting is not a DIA acquisition [[yet).
And then there are the Van Gogh's at the DIA. The DIA's finest Van Gogh is a portrait of Joseph Roulin [[gift of Josephine Clay Ford originally in the Ford House in GPS)...
http://en.wikipedia.org/wiki/File:Portrait_of_the_Postman_Joseph_Roulin_[[1888)_ van_Gogh_DIA.jpg
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