1. Childe Hassam, Flags, Fifth Avenue, 1917
This scene of Fifth Avenue by American Impressionist Childe Hassam was one of 30 such images showing the flag-bedecked thoroughfare during World War I. The canvas captures the dynamic verve of New York just as the country began its ascent to superpower status. Bequeathed to the Metropolitan Museum of Art by its former owner, Brooke Astor, the work disappeared in 2002 when her son wrongfully sold it to private hands for $10 million. Its whereabouts remain unknown, even though it’s one of the most famous American paintings of the early 20th century.