Dear …

I want to take this opportunity to say how impressed I was with the way you __(performed on /handled)__ the __(type of project)__. Your efforts were outstanding and I found you to be exceptionally helpful on this __(case/project/etc)__. You were willing to help in any way and made my job a lot easier.

I was impressed by your willingness to take the time to review and understand the instructions well before starting each task. Your involvement was both thorough and efficient. And you demonstrated that you are a hard worker with __(briefly describe something that stood out)__. All this instilled great confidence in me with the suggestions you offered.

It is a pleasure to work with you, Name. You are an excellent representative of __(name of organization)__ and I commend your performance. Well done!

Sincerely,