Spirit and Frontier.Carlos Yudica/Marcus Mainka/Shutterstock Frontier was named the worst US airline in 2023 by claims management company AirAdvisor. Spirit, another low-cost airline, was ranked the second-worst on the list. Delta took the top spot as the best US airline this year. AirAdvisor, a Boston, Massachusetts-based claims-management company, published its first-ever top 10 US airlines rankings on July 20.