If you are in Thailand, you could choose to skip any attraction but not the ancient city of Ayutthaya. A UNESCO World Heritage Site, fully deserving of its status, it was the Thai capital for 417 years.
The city is an open-air museum of art and architecture. Founded in 1350 by King U-Thong when the Thais were forced southwards by their not-so-friendly northern neighbours, Ayutthaya was the seat of power for 33 kings and several dynasties before it was sacked by the Burmese in 1767, leaving the city ruined and abandoned.
It has since then been rediscovered, and is now a bustling town where the modern and ancient blend well. The city is dotted with structures going back centuries, and all marvels.