Sunset viewing is one of the things I enjoyed most in my tour whether local or abroad. And I was given the opportunity in Siem Reap to shoot a sunset abroad. Last time I did this was in Phuket, Thailand.
If you are going to have this tour in Siem Reap, the first advice/tip is to come early on the site. It is better to be at the top of the mountain at exactly 5 pm due to the multitude of tourists.