My husband and I spent 3 days with San Park in May and we can't say enough good things about him. We found him through a thread in this forum, emailed him, and set up our trip promptly.
When he and a driver (Yun, I believe) picked us up from the Siem Reap airport to transfer us to our hotel, San began telling us about the area and temples on the car ride. It was immediately clear that he loves his job and is extremely knowledgeable about the history of his country. Beyond that, he's just a really likable guy - very down-to-earth, funny, and nice as can be.
Over the 3-day trip, San took us everywhere and anywhere we wanted to go, even stopping at a local market to help us haggle for a hammock! With San, we were able to do as much or as little as we wanted to do. "Up to you", he would say. Plus he took pride in the photos he took of us, making sure to get the best picture possible. He also pointed out to us all the perfect photo opportunities, which we appreciated.
We got to do so much with San that we wouldn't have with another tour guide, like walking through the forest to get from one place to another, walking on some precarious paths for adventure ("Do you want to rock and roll?", he'd ask).
If you're taking a trip to Siem Reap, don't hesitate to contact San Park to be your tour guide. He'll make your trip so enjoyable. We're so glad we found him!
You can find his contact info here - http://sanpark.angkor.googlepages.com