Americans love their plastic. Last year, for the first time in the Total System Services U.S. Consumer Payment Study's history, credit took over the top spot as the preferred way for U.S. consumers to pay for their goods and services. The survey found respondents liked their credit cards for a variety of reasons including fraud protection, rewards, and payment flexibility.

Investors and consumers alike would be wise to seek out which credit card companies are most beloved by their cardholders. J.D. Power, the consumer advisory service, recently conducted a study of almost 23,000 credit card account holders to determine which credit cards had the most satisfied customers. It measured customer satisfaction by examining six factors: interaction, credit card terms, billing and payment, rewards, benefits and services, and problem resolution.

While several banks and financial institutions were included in the study, two stood out from the rest, scoring the full five Power Circles from J.D. Power. American Express Company (NYSE:AXP) took the top spot, followed by Discover Financial Services (NYSE:DFS). No other credit card company scored five (the highest score) -- or even four -- Power Circles in J.D. Power's study. Let's take a closer look at what these companies might have done to please their customers and whether this offers insights for investors.

Three credit cards shown fanned out: one American Express card, one Visa card, one Mastercard card.

Which credit card company has the most satisfied customers? Image source: Pixabay.

What American Express is doing right

American Express has been working overtime to right the ship since being dumped last year by Costco Wholesale. One of the things American Express has worked particularly hard on is increasing its merchant coverage.

Through its OptBlue campaign, the company's marketing outreach to smaller merchants who do not yet accept its cards, American Express now offers flexible pricing, a listing on AmEx's Shop Small map, free online ads, and American Express signage. In the company's second-quarter conference call, CFO Jeffrey Campbell maintained that the goal is to reach domestic merchant coverage parity with other credit card networks by 2019.

What probably wins American Express the most loyalty from its customers is the rewards offered to its cardholders. Surveys show that consumers increasingly believe rewards are the most important criteria when selecting a credit card. In response, American Express has spent ever larger amounts on reward expenses to keep its customers happy.

This quarter, the company's card member reward expenses increased 9% year over year to $1.9 billion. The strategy appears to be working, as management says its focus on reward categories that competitors have a hard time matching, like increased airport lounge access, have kept account holders in the fold.

What Discover Financial is doing right

In Discover's first-quarter conference call, taken from the transcript provided by S&P Global Market Intelligence, CEO David Nelms summarized what Discover card holders like about the company so much: "Customers ... recognize the benefits enjoyed by all Discover it cardholders, including award-winning, all-domestic customer service, innovative security features, free FICO scores, no annual fee and a competitive rewards structure."

One example of the company's "innovative security features" is its "Freeze it" mechanism. In what essentially amounts to an on/off switch for a credit card, this feature allows card holders to immediately freeze their accounts via a mobile app or online if they have misplaced their credit card.

Nelms emphasizes that rewards are important and believes the 5% rewards that account holders earn on revolving categories engages its customers with their cards.

However, Discover management insists it must offer a more holistic experience to its customers than competition in an arms race over which card company can provide its members the most rewards. In 2016's fourth-quarter conference call, Nelms said:

You need to be careful ... not to get hung up with just rewards. It is more than just the rewards. It's great service. It's other features. It's how you work with credit to make sure you're going to people that don't get overextended. There's a bunch of pieces here and rewards is an important but just one piece of the puzzle.

Discovering a great way to Express returns

Customer loyalty is a great way to measure if companies are offering valued services or products. When companies score high marks in this traditionally hard-to-quantify metric, it's unlikely that customers will abandon the service or experience anytime soon.

What these consistent high marks from these two companies show investors is that, regardless of other struggles they may be experiencing, they are hitting the one mark that is absolutely necessary if they want to thrive in the years ahead.