Phnom Bakheng is often described as the best spot to experience a true Angkor sunset. Apparently, every person in the world knows about that, because it's full of people from 4 to 7pm. Be there early, and you might get a place where you can actually see the sun setting over Tonle Sap lake. Be there late, and you'll only see the backs of thousands of people. Sunset on Phnom Bakheng is not really recommendable if you're looking for a romantic thing to do with your girlfriend, but if you want to take some interesting photos, this is the place to be. First, you can watch and document people's strange attempts to climb up the steep stairs of the temple. Second, you can watch the crowd go crazy when the sun approaches the surface of Tonle Sap lake, illuminating it magically with its intensive orange light. They will even start applauding! Third, and this is the no-nonsense thing about sunset on Phnom Bakheng, you can take photos in the best light conditions when everybody else is hectically descending down the hill and the temple empties in some minutes. At 6pm, there are 1000 people, at 6.05pm, there are 500 people and at 6.15pm it's just you and some others.
For security reasons, the authorities have closed the very steep and rocky direct way to Phnom Bakheng. Some years ago, it was terrible to climb down a landslide-like hill in complete darkness. Nowadays, a winding path leads along the hill. It is not lit either, but at least there are no rocks on the way. Alternatively, you can ride up the hill on the back of an elephant. For either method, plan at least 20 minutes up the hill.
Phnom Bakheng closes at 6.30pm.