Aya Sofia in Istanbul. Not just a beautiful building, but living history, and a Christian domed church that ironically set the standard for all Mosques built after.
La Mezquita, the great Cathedral of Cordoba with its myriad of arches and pillars.
The Parthenon, because it set the standard for Western Architecture for millennia afterwards, and because of its elegance and visual impact.
The Great Amphitheater of Ephesus.