What is more important, a product of good quality or excellent customer service?

excellent customer service will push the customer to buy the product even when the product is a trash however the opposite is never true that's to say a good product cannot sell itself unless an excellent customer service is provided .

Excellent customer service makes a difference in many situations, but not all of them.
Strategically, companies may choose between three 'contradicting' directions: customer service, innovation and efficiency.
These are 'contradicting' to the extent that companies cannot excel simultaneously with the three of these and must chose their field to make a difference.
Companies focusing on pure efficiency may give poor-to-average customer service and still be successful, since their value proposition is all about being the cheapest in their field.
Did you ever receive (or expect) good customer service from low-cost airlines? And bad words about their service don't prevent them from flying, do they? Companies focusing on pure innovation (technological, process, business innovation, does not matter) may give average customer service and succeed with a value proposition that is all about uniqueness in the market.
Sustainability is at stake, even the most innovative (and successful) product gets copied one day or the other, losing its uniqueness.
Mainstream companies competing in mature markets tend to chose one of these strategic directions, and combine this with good-to-excellent customer service, since the difference on products (or product quality) is more and more difficult to perceive.
And this is a financially deadly game, when excellent customer service is offered throughout the market, how far do you want to go to serve your clients? Excellent books about this debate: The Discipline of Market Leaders, Treacy & Wiersema, 1997 Blue Ocean Strategy, Kim & Mauborgne, 2005

In my opinion and as per my understanding after working so many years in front line sales.
I come to conclusion both are like wheels of same vehicle and you can not manage to run vehicle if not either on their proper place.
Though a good and prompt customer service on both secondary and primary level do overcome some of the flaw in the quality of product either by timely replacement or addressing the concern issue in prompt manner.
I believe in today's scenario where business are more complex and not limited to one ethnic group to one city to one country or one continent.
The opinion may defer and one may suits and result better in absence of other but broadly Quality product is very important if you enter in the new market place and where competition is stiff and that always helps excel because quality not go unrecognized for so long.
Cheap product may make big splash but can't help sustained for so long and eventually bad words spread about the product and it's difficult to turn the tide.
Customer service can't help you in case where customer is not satisfied with the product itself.
So I believe Quality Product seems more important then backed by decent customer service.

If the Target is "continuing" : then, Both of them crucial: they combined together as one thing. 
 

An excellent service, customer is always attract by product but always conveyance by a good sales tactics or good customer service.

Good Customer Service will definitely help to sell even a low quality product and bad service may even produce difficulty to sell a good quality product.
so Customer service has vital role in every case.

Good quality obligates customers to buy without services

Clients do not only buy products, they buy emotions.
That's why the focus on customer service has become a lot stronger in the last decade.
Ccompetition has become harsh; quality differences are not so big anymore (e.g.
in smartphones) and the competitors provide more and more similar features in their products.
That's why the advertising, the marketing and the sales strategies of a company provide much more creative fields to stand out, to make the company's products unique, to argue that the company XY is the one to go for.
Excellent customer service is therefore one of the most important pillars in product-oriented businesses.
http://www.tomhopkins.com/blog/presentation/arouse-emotions-dont-sell-logic

Excellent customer service