The coronavirus pandemic upended air travel around the world, from half-empty planes and canceled flights to rapid Covid-19 tests and mask requirements.

But at least one thing hasn't changed: Qatar Airways is once again the "world's best airline," air transportation research firm Skytrax announced on Tuesday. Qatar Airways also topped Skytrax's global airline rankings in 2019 — the firm skipped its annual ranking in 2020 — making the Doha-based airline a repeat champion, and a six-time winner over the past decade.

This year's rankings are based on more than 13 million customer survey responses, judging 356 airlines on areas like online booking and check-in processes, friendliness of airline staff, seat comfort and the quality of in-flight meals and entertainment. The survey ran from September 2019 to July 2021, according to Skytrax.

Delta Air Lines won the title of best North American airline, while Japan Airlines ranked as the world's best economy class. The survey also recognized Malaysia's AirAsia as the world's best low-cost airline.

Here's a breakdown of Skytrax's 2021 airline rankings, both globally and in North America:

The world's top 10 airlines of 2021

Here are Skytrax's top 10 airlines of 2021, by overall rating:

1. Qatar Airways
2. Singapore Airlines
3. ANA All Nippon Airways
4. Emirates
5. Japan Airlines
6. Cathay Pacific Airways
7. EVA Air
8. Qantas Airways
9. Hainan Airlines
10. Air France

Qatar Airways earned especially high marks from business travelers, topping all other airlines in business class-related categories like best seat, best airline lounge and best onboard catering. Travelers rated the airline third for in-flight entertainment, behind Emirates and Singapore Airlines, and fourth in airline cabin cleanliness.

But the reigning champion's dominance isn't without controversy.

Over the past decade, Qatar Airways has come under fire for its treatment of women employees, including contract provisions forbidding its mostly female cabin crew members from getting married or pregnant within five years of starting work with the company. The airline defended those practices in 2014, but then phased out those restrictions a year later.

Women passengers have also reported issues with their treatment on Qatar Airways flights. Last October, 18 women were subjected to invasive and compulsory medical examinations while traveling from Doha to Sydney. Weeks later, the company said it regretted "any distress or infringement" caused by the incident.

Despite that incident, the airline has racked up accolades since the pandemic hit. In July, Qatar Airways nabbed the "best airline" title from airline safety website AirlineRatings.com, which praised the company for adding new destinations during the pandemic. Many other airlines have cut down on flight routes to make up for massive revenue losses.

It's hard to tell how much the move helped: Qatar Airways reported a $4.1 billion annual loss on Monday, and a drop in passengers of more than 80% over the past year.

North America's top 10 airlines of 2021

Here the top 10 airlines in North America, according to the ranking:

1. Delta Air Lines
2. JetBlue Airways
3. Air Canada
4. Southwest Airlines
5. WestJet
6. Air Transat
7. United Airlines
8. Alaska Airlines
9. Hawaiian Airlines
10. American Airlines

Delta and JetBlue both jumped ahead of Air Canada, ending a streak of three straight North American titles for the Canadian carrier. But even the region's top-ranked airline wasn't immune to the pandemic's financial impact: Delta lost a record $12.4 billion loss last year, part of a $35 billion loss for U.S. domestic airlines in 2020.

The Atlanta-based company has somewhat bounced back recently, reporting a quarterly profit in July — its first since the start of the pandemic — due partially to federal coronavirus aid. Delta also reported that demand for domestic leisure travel is back to pre-Covid levels, with business travel still recovering.

In August, Delta CEO Ed Bastian told CNBC that Delta's flights were more than 90% full during a recent weekend, as people learn "how to manage and live" with the pandemic. The carrier has publicly focused on getting as many of its employees vaccinated against Covid as possible, announcing last week that 82% of its employees are now fully vaccinated.

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