Dear …
I want to take this opportunity to say how impressed I was with __(name)__ as __(he/she)__ helped me to __(briefly describe what he/she helped you to do)__. I found that__(he/she)__ was exceptionally helpful; willing to assist me in anyway __(he/she)__could.
__(Name’s)__ dedication and efforts are nothing short of exceptional. __(He/She)__ took the time to review and completely understand my __(project/case/situation/etc.)__. __(He/She)__ was both thorough and efficient, which gave me full confidence in the suggestions __(he/she)__ made. In addition, __(he/she)__ proved to be an excellent liaison between __(name the parties involved)__ and did whatever __(he/she)__ could to expedite the process.
I want to commend __(name)__ on __(his/her)__ outstanding efforts. __(He/She)__ is an excellent representative of __(name of organization)__ and is a pleasure, indeed, to deal with.
Sincerely,