Angkor Wat was built by King Suryavarman II in the early 12th century as his state temple and capital city. As the best-preserved temple at the site, it is the only one to have remained a significant religious center since its foundation – first Hindu and then Buddhist.
One of the amazing things about Angkor Wat, aside from the structure itself, is the 1000+ square meters of bas relief ringing the inner structure.
One other note: the sunrise from Angkor Wat is reportedly amazing. In four days, we didn't make it there for sunrise...two of the days were cloudy (one with rain), and on the other two, we were just too tired to get up that early due to a missed connection and unexpectedly long trip getting to Asia in the first place.