Dear …
I want to take this opportunity to say how impressed I was with the professionalism __(Name)__ demonstrated as __(he/she)__ helped me to __(obtain the appropriate permits/documents/etc.)__ for __(construction of .../resolving my type case/etc.)__. Until I encountered __(Name)__, regrettably, I had experienced __(length of time)__ of significant frustration with your __(name of department)__. I can’t tell you how refreshing it was to deal with an individual like __(Name)__; someone who took the time to review and understand my __(project/case/situation/etc.)__.
__(His/Her)__ dedication to __(his/her)__ job is nothing short of exceptional. I found __(him/her)__ to be tremendously helpful; willing to assist me in any way. __(He/She)__ was both thorough and efficient, doing whatever __(he/she)__ could to expedite the process. I commend __(Name)__ on __(his/her)__ outstanding efforts. __(He/She)__ is an excellent representative of __(Name of organization)__ and is a real pleasure to deal with.
Sincerely,