Heard from an acquaintance that the Angkor -- Siem Reap tour guide we recommended did a good job for her companion and her on a recent trip.
The guide is:
(Mr) Darith Touch
phone +855 12 793 908
English speaking for sure. I see from his business card that he speaks Japanese, too, but I can't vouch for that.
What I can vouch for is good English skills, a good ability to arrange for transport and accommodation. He's honest and hard working. I've known him for some years now and I find him thoroughly reliable.