Just a short note to express to you my sincere thanks for the fantastic result.
From the outset I indicated my appreciation of your professional excellence, cool, calm and dispassionate judgment, prompt efficiency and infinite patience. I could go on and on but I better stop at this as otherwise it may sound that I am turning you into a saint!
The result was beyond my expectations and was a result of your hard work and professionalism combined with the stubborn perseverance of [our secretary].
With my sincere appreciation and best regards,