The famous UNESCO listed Angkor Wat temple in Cambodia’s northern province of Siem Reap was originally built as a Hindu temple and then converted to a Buddhist, it functioned as the centre of the Khmer Kingdom. The tourist crowds get large here, but at the end of the day it is not too bad. It represents one of the largest archaeological sites in operation in the world and, to be fair, they do a pretty good job of managing the tourists. In places they have installed fences, and even wooden steps to protect the centuries old structure underneath.
You must have a registered guide, it’s about $20 per day for a ticket to anywhere within the whole complex.