I used janthana "Jan" samleerangkul <
[email protected]> before and my wife and I enjoyed our time with her. She was generous with her time and took us to all the places we requested and was fun to be with.
I later recommended her to some friends who were visiting Thailand for the first time and they also gave her a thumbs up.
Our target was to visit some fairly offbeat places and had a pretty good idea of the types of places we wanted to visit and the things we wanted to do. If you don't speak Thai it can also be a help to have someone willing to negotiate for you at some of the markets, etc.