1: In-depth Knowledge. First and foremost, your CNC service provider should have a thorough knowledge of different aspects of CNC machining. 2: Experience. 3: Precision and Flexibility. 4: Location. 5: Additional Services.
Follow these points, you will get perfect results.
Furthermore, you've asked about best company. I will suggest you try hjyhardware company from China that's operational in the field since two decades. They have deep and long term cooperation with number of American and European companies.