In a year when airlines were forced to defend themselves and their service following several high profile incidents where passengers and flight crews clashed, Alaska Airlines managed to excel, according to the annual Airline Quality Rating.
The carrier topped the list for a second-straight year, narrowly edging out Delta Airlines.
"Alaska and Delta are so tight together, we had to take it to three decimal places just to see who won," said Dean Headley, a professor at Wichita State University. Headley and Brent Bowen from Embry-Riddle Aeronautical University are the authors of the Airline Quality Rating, which calculates the performance of U.S.-based airlines using four factors measured by the U.S.
Department of Transportation: percentage of bags mishandled, on-time arrivals, denied boardings and complaints to the DOT.
Alaska delivers consistently high service, according to the data, Headley said.
"Alaska has pushed over the last year or so ... to do everything right. They want to be the best airline, and it shows. The intent is there and the performance is there," said Headley.
The top 12 airlines in the study are:
For Spirit, dropping one spot to last in the Airline Quality Rating is a reflection of its customers complaining about service.
"The thing that drags them down year after year is the fact their complaint rate is about 5.5 for every 1,000 passengers and the industry rate is 1.3," said Headley.
Spirit Airlines said it is "committed" to improving reliability, guest service and the in-flight experience, and it noted that its DOT complaints fell by nearly 80 percent since May 2017.
"It's also important to note the Airline Quality Report doesn't factor other things that Guests love about Spirit, including the lowest fares in the industry and newest fleet of planes in the U.S.," Spirit said in an email to CNBC.
Overall, the study shows the quality of airline service hit a record high last year due to carriers posting record low numbers for mishandled bags and for passengers being involuntarily bumped from flights.
In fact, denied boardings were down last year from 2016 as airlines changed their policies after a passenger was dragged off a United Airlines plane. Video of the confrontation prompted a public outcry for airlines to drop, or dramatically reduce the practice of overselling flights.
"We hope the industry got the message," said Headley. "It was a horrible message that we saw last spring. Certainly, from that point in time through the end of 2017, the industry as a whole cut its involuntary denied boardings."
Also Monday, TripAdvisor released its rankings of the world's best airlines. It ranked Singapore Airlines at the top. Southwest was the only U.S. airline to make the list, which was compiled from reviews by passengers.