You can’t go wrong with Asia Tour Advisor!
I was in Cambodia with two friends – all of us women traveling in our 20s. Asia Tour Advisor was recommended to us by friends, and by the time we finished our two days there, we had already passed their name on to 3 other groups who all used them within the next month. The provided guide not only has incredible knowledge of a huge range of sites, but also was able to create a truly personalized experience for us. He helped us identify the ruins and other activities that would be most interesting to us, he provided customized tours of the ruins, and he also shows us some more local/less touristy locations and restaurants that gave us a true sense of the country. We couldn’t have had a better experience, and only wish we could give more than a 5/5 rating!