Twenty-four airlines were rated on how often they award free tickets in response to website inquiries on redeeming frequent-flier miles. IdeaWorks evaluated the carriers by making 6,720 web requests for a standard-mileage award.
Dallas-based Southwest awarded free seats 99.3 percent of the time, the study showed. Worldwide, only Brazil's GOL awarded seats more often — 100 percent of the time.
United Airlines ranked third in the U.S. and 11th worldwide for its frequent-flier generosity, with an award rate of 71.4 percent, according to the IdeaWorks study.
The stingiest carriers in the world? According to the study, the worst is US Airways, with an award rate of 25.7 percent, followed by Delta Air Lines, at 27.1 percent.
The average award rate for U.S. airlines was 68.6 percent, up from last year's 66.1 percent rate.