ValuePenguin studied customer complaints reported to the federal Consumer Financial Protection Bureau regarding the six largest credit card companies. The credit card researcher then weighed the complaints against the companies' 2013 card purchase volume to come up with a "complaint metric," or score, for each company. The average score was 5.22; the lower the score, the higher the satisfaction. Complaints reported directly to the companies or to other consumer agencies were not included. American Express, with $637 billion in charges, received a complaint score of 1.59 for 1,014 complaints per $1 billion in charges. Top three complaints: Billing disputes, account closure, rewards.