The best customer service is that which suprises you, when someone has clearly surpassed what is expected, and delivered above and beyond. When it is clear that this is company policy as opposed to a "rogue" individual, then an organisation will be recognised as providing great customer service.
There are formal measures of customer service, such as the net promoter score, which is based on on the simple question "would you recommend company X to a friend". For me, the best examples of customer service are those which are passed on via word of mouth, such was the level of delivery to the customer.
My best example was a train journey, where I was delivered a fantastic level of customer service. I arrived early for my train, but had to queue for the ticket machine, and when I arrived at the front of the queue I could not understand which ticket to buy, there were so many prices and options. After eventually purchasing a ticket I found the train, but therer were no seats left so I had to stand. The train left 10 minutes late due to a signalling problem, and the air conditioning on my coach was broken and their were no opening windows, so it was boiling hot. I went to the catering car to get a drink, and found that the train company policy had changed and they no longer took credit cards. Sensing my growing frustration, the operator in the catering car gave me a free drink and wished me a pleasant journey.
So from this nightmare journey, what do I think of the company and its customer service? I dont tell people about all the problems, just that the company was forward thinking enough to give their staff the power to deliver great customer service. Clearly the employee would not have to explain to the manager why drinks were being given away, or the manager had actually issued instructions to make sure that harrassed passengers were being looked after. Now anyone I meet will hear this story, and my positive views on the company, whenever the organisation in question is mentioned. I am a "net promoter".
Maybe that is the answer to great customer service, its not always delivering the best service, or the cheapest product, but how you deal with the problems that arise, and the culture embedded in an organisation that only comes to light under pressure - that tipping point where a tiny decision one way or the other can be the difference between a great customer service experience and a terrible one.