Based on which criteria, we can accept a team leader as a best customer service leader. In other word which requirements must be fulfilled to become a best customer service leader.

A good leader always leads by example - a good Team Lead must also do the same. In a way, you can measure the Team Lead by watching how his Team members react to him - if he/she can gain their confidence and loyalty you can pretty much expect him to lead in providing good customer service as well - but then, there might be exceptions !

Suraya, I think part of the answer has to do with what your (larger) organization needs to achieve and what the area called "customer service" needs to provide, and then there is a "best customer service leader" within that group. Something I have noticed over the years is that there is often a temptation in some contexts to equate customer service as a function with a lower skill area in what a company does. What I have also noticed is that various departments within one company also want to claim "customer service" as a success measurement owned by them. (Customer service is held much more dearly in those instances.) Is this for "the Customer Service Department" or is this for " someone representing what it means to deliver for our customer, no matter where you are in our company"? So, lots of different ways to look at your question. When you remove yourself from all of the political things to consider, you can look at what needs to be done to create a successful outcome. Who creates success for a person/group in need of something, and who does this on an ongoing basis, so that a client is in business with us and stays in business with us?