Discover Financial Services topped peers in a J.D. Power credit-card customer satisfaction ranking, ousting American Express from the No. 1 spot for the first time in the survey's nine years.

Amex, the biggest credit-card issuer by purchases, slid to second place after it tied for first with Discover last year, according to results released Thursday. Discover captured the top spot with 828 points in a 1,000-point scale, the survey's highest score ever, according to Jim Miller, senior director of banking services at J.D. Power.

"Discover just regularly keeps coming up with new ways to increase customer satisfaction," Miller said in a phone interview. "They execute really well, they keep problems to a minimum and they're constantly coming up with new ways to make customers' lives a little bit easier."

The ranking is the latest blow for Amex, whose executives have touted previous J.D. Power dominance as proof of competitive strength in customer service. Early this year, Amex announced an end to partnerships with Costco Wholesale and JetBlue Airways. The New York-based card company also has endured legal woes over an antitrust dispute with the U.S. Department of Justice and a rejection of a settlement with merchants over fees.

"Amex is still very solidly in second place," Miller said. "There's a very different business model at Amex. It's a much more segmentated model with different cards for different people."

JPMorgan Chase remained in third place while Capital One Financial climbed to fourth, the biggest mover after placing eighth in 2014. Citigroup, the world's biggest credit-card lender, ranked second-to-last out of 10 companies, ahead of private-label card issuer Synchrony Financial, which is being broken off of General Electric.

The survey looked at credit-card terms, rewards, billing and payments, interaction, benefits and services, and problem resolution. Riverwoods, Ill.-based Discover topped all six categories. The marketing information company's study included responses from more than 20,000 credit-card customers, and was fielded from September to May.

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