If I had to pick the historical site with the most ambiance, the most picturesque vistas, the most fascinating historical stories, the most amazingly unique architecture, Angkor in Cambodia would be it. I lost count of how many amazing individual sites that I visited there that would each have been worthy of World Heritage status in their own right, Angkor Wat itself actually only being one of a complex of many. At times Hindu and Buddhist, it is the most famous of all Khymer architecture, and was built between the 9th and 15th centuries. Other important and memorable structures include the Angkor Thom (the capital area itself), the Terrace of the Elephants, the Ta Prohm, Preah Khan and the similar Prasat Banteay Kdei, and many smaller temples interspersed with Baray (large water pools) including the massive Srah Srang as big as a small lake, Phnom (hills), and naga-bridges with stone guards lining either side. Kbal Spean had amazing carved rocks on the creek bed, and there were even pyramids like Phimeanakas.
I think my favorite complex was the Prasat Bayon, which has 216 stone heads around 2 meters high each, peering eerily down at you, seeming to follow your movement from all angles. Sometimes mistaken as pure representations of Buddha, they were actually modeled on one conceited king (Jayavarman VII) as the Buddha Bodhisattva Lokesvara.
A great map of the whole complex can be found here.
"The area of Angkor has many significant archaeological sites, including the following: Angkor Thom, Angkor Wat, Baksei Chamkrong, Banteay Kdei, Banteay Samré, Banteay Srei, Baphuon, the Bayon, Chau Say Tevoda, East Baray, East Mebon, Kbal Spean, the Khleangs, Krol Ko, Lolei, Neak Pean,Phimeanakas, Phnom Bakheng, Phnom Krom, Prasat Ak Yum, Prasat Kravan, Preah Khan, Preah Ko, Preah Palilay, Preah Pithu, Pre Rup, Spean Thma,Srah Srang, Ta Nei, Ta Prohm, Ta Som, Ta Keo, Terrace of the Elephants, Terrace of the Leper King, Thommanon, West Baray, West Mebon."