The Killing Fields outside Phnom Penh.
This is a very sad and haunting place yet I am very glad that I made the trip out there. Not to do so would have felt like I was dishonouring the memory of the thousands of poor souls who so cruelly lost their lives there.
The grounds themselves are very well preserved and there is an excellent audio tour which explains in a lot of detail what happened at the site. I recommend taking the trip even if it is a difficult experience.