Bakheng Hill
It is a great place, because the temple it's on the top of hill. That way you'll get an unobstructed view of the sun as it get down.
It is also a great place, because what could be more Cambodian than watching the sunset from an ancient Cambodian temple?
As a bonus, you'll also get to see the Angkor Wat complex from a distance, and at high elevation.
And finally, when you are at the top of the Phnom Bakheng temple, you have the enviable position of being able to see all of the landscape just by walking around from one corner to another. And the view is really pretty and you never see the great place like it before.
+ The ways that you can
As always, tuktuk is the transportation of choice in Angkor Wat and Siem Reap. It should be around $5 if you’re going from the hotels in Siem Reap. You can just say to your driver that you want to see sunset at Angkor Wat, and the default location will be Phnom Bakheng (doesn’t never show you how to see famous it is?).