You can't pick just one. My own favorites:
The Parthenon. Chartres cathedral, particularly the Rose window. Someone mentioned the Cro magnon cave paintings. Amazing. Love those geometric Islamic tile decorations.The Alhambra. St Basil's onion domes. It may be trite, but yes, the Taj Mahal. In its day it must have been fantastic. For all my un-belief, there is no doubt that many religions have left us with some truly priceless things.