Which is my favorite temple ruins among all the Angkor temples that I visited?
It is Preah Khan.
Unlike others which are mountain temples, Pheah Khan is a collection of chambers and open space. And unlike others which at once impose their presence upon sight, Preah Khan is unassuming. Its entrance is an open courtyard with statues of devas and demons lining up on both sides. That leads to an door or archway which in turn leads you to the temple itself.
You will only realize its vastness once you are inside. Indeed, I was lost within the temple ruins after wandering with my camera. Its greatness is subtle and I like subtlety.
Pheah Khan is also interesting to me because it is the clearest example of Hindu-Buddhist conflict in the past. The Khmer Empire was primarily a Hindu polity but for some decades, its rulers decided to adopt Buddhism. The Hindu reaction came later when its rulers embraced Hinduism again. It was during this time that various Buddhist images were vandalized or redone as Hindu icons. In fact, Hindu kings converted Buddhist temples into Hindu temples.
Here is an example of Hindu reaction to Buddhism in the Khmer empire; images of Buddha were chiseled out:
Here is another where a Buddha image was remodeled as a Hindu holy man:
