What is the most important factor in delivering impeccable customer service?

In my opinion; - Never make a promise that you can not keep, in full.
- If you say the repair man will be there at a certain date, and time, please make very sure that he is there then.

There are many variables that need to be taken into account to deliver outstanding customer service.
Basic marketing principles such as know your client and present products and services that fulfill the needs of your clients better than competition are key in arriving at outstanding customer service.
I would summarize all of these into what I call the motherhood effect.
Treat your clients as you would your own mother.
Of course there will be exceptions to this rule as the analogy will always have its limits.
However, the basic principle of creating an environment where a company will analyze, develop, produce and distribute their products and services to their target market as they do for their own mothers, the results will be tremendously successful.

Delivering an impeccable product!

proactive Communication & coordination.
Keeping them informed about the status of their requirement

Be loyal with customer, Always available to solve there problem.
and listen them first...