The 2nd cave of Dambulla
The Cave of the Great Kings (also known as the Maharaja Vihara) is the second and most spectacular of the caves at the Dambulla Cave Temple, or Golden Temple, complex in Dambulla, Sri Lanka. Legend has it that the cave was originally built in the 1st century BC by the King Vittagamini Abhaya (thus the name), but the majority of its fine and famous frescoes were added in the 1700's at the behest of the kings of Kandy.
The cave is over 50 meters long and contains over 55 separate Buddha statues as well as representations of Hindu deities Saman, Vishnu, and Ganesha, a common occurrence in the melting pot which is Sinhalese Buddhism. Rather than drone on about the history and technical aspects of the cave, I just want to focus on the frescoes, which are one of the most incredible things I've seen in all my travels. The entire cave ceiling is covered in ornate and detailed frescoes outlining everything from the Buddha's birth to his temptation by demons to his attainment of enlightenment, and every nook and cranny in between is filled with beautiful geometric designs. The frescoes are remarkably well-preserved and still retain their eye-popping colors (so, no flash photography, please).