It's Spirit. JK, it's actually Alaska Airlines!
JD Power released its Airline Customer Satisfaction Report for 2015, assigning domestic carriers a score on a 1,000-point scale that accounted for seven different factors ranging from cost and fees, to in-flight services, to the check-in experience. Alaska took the top honor in the "Traditional Carrier Ranking" with a score of 719, while Delta and American slid into second and third with 709 and 700 points, respectively.
The winningest carrier in the west also took home the gold in JD Power's Loyalty/Rewards Program Satisfaction Report, scoring a 759 on the same 1,000-point scale, accounting for factors like ease of redeeming points/miles, reward program terms, and customer service. Alaska's lead was even narrower here, though, beating out JetBlue Airways by only a single point.