Rockefeller Center
Actually a 22 acre "city within a city", Rockefeller center was built by John D Rockefeller Jr at the height of the Great Depression. It was the US's first retail/entertainment/office center of 19 buildings and declared a National Landmark in 1987. Most interesting in this space is the GE Building offering a Top of the Rock observation tower (actually taller than the Empire State Building's) and Rockefeller Plaza.