This was my third trip to Siem Reap and the first to have good light. Having been there in the hot season and the wet season I recommend the "cool" season, with January the best for photography. There is a very short "golden hour" in the morning of about 20 minutes and you need to get there about 5:30am to get a decent location. There are still 100's of people around at that time, with flash popping all around - getting an interference-free shot isn't that easy.