What is the most professional objective for the customer service that I can add to my CV?

If you have a credit of Excellent Customer Experience rendered, add to that with referral of customers who had enjoyed such experience with you. 

If your customer services are being measured by your organisation, then you shall set an objective desirable.

For example - Individual Customer Experience Score vs Company's and region's to that of yours.

You can add Time efficient, multitasking, team spirit oriented.