In the Mood by Glenn Miller (1939)

This is the defining song of the Big Band and World War II eras. The song was released right before the Billboard charts debuted, so the song’s true popularity can not be measured in comparison to others. Undoubtedly, "In The Mood" is one of the most important works of the twentieth century.

http://www.youtube.com/watch?v=teJfuKdzbOo