For a true lost temple experience, Koh Ker temple ticks all the boxes. Located in Preah Vihear province, but easily accessible by road about two hours from Siem Reap, it is far from crowded. Usually, only a handful of locals can be found exploring the ancient site, making a visit super special.
The unique pyramid structure is unlike any other temple in the Angkor temple complex. Koh Ker is a former ancient capital of the Khmer Empire before it later moved to Angkor.
It is still hidden deep in the jungle, giving a visit a real explorer feel, and only the main temple and a few surrounding monuments are easily visited. As of 2021, Koh Ker is awaiting final discussions and registration as a UNESCO World Heritage site.
A visit to Koh Ker can be combined with Preah Vihear temple and Beng Mealea as a long day trip from Siem Reap.