The last of the places Tori recommended to me, Beng Mealea temple was on of the most overgrown and one of the largest temple complexes I visited around Siem Reap.  Built in the early part of the 12th century, it looks almost as if it has been deserted since the day it was built.  Though pretty crowded, it was entirely acceptable to crawl in and over the ruins to access different parts of the temple, which was a lot of fun.