Angkor Wat

Among the wonders of the world, this is the main temple in the ancient kingdom of 100s of temples spread out in the rainforest.

As the most famous it is also the most touristed, so go very early or late. And make sure you visit a tiny temple (ask your local tuk tuk guide) which is not so popular - you're likely to have it for your self.

Wikipedia: The temples of the Angkor area number over one thousand, ranging in scale from nondescript piles of brick rubble scattered through rice fields to the magnificent Angkor Wat, said to be the world's largest single religious monument. Many of the temples at Angkor have been restored, and together, they comprise the most significant site of Khmer architecture. Visitor numbers approach two million annually.
In 2007, an international team of researchers using satellite photographs and other modern techniques concluded that Angkor had been the largest preindustrial city in the world, with an elaborate system of infrastructure connecting an urban sprawl of at least 1,000 square kilometres (390 sq mi) to the well-known temples at its core