What is the best way to complain to an airline?

If the passengers face any issue with the airline's customer service team, all they need to do is find a way to get their complaint registered with the airline. The problem can be about anything you have faced during your flight or when making the reservation, or it can be a complaint about airline refund and cancellation of your flight ticket. The complaint form is the easiest way to get compensation and a refund for your flight ticket.

Jump onto the airline's official website through which you have made the reservation.
Once you get onto the homepage, look for the 'Contact us' option at the end.
Once you get onto the contact page, you are required to look for the Complaint form.
Fill out the details asked in the form, which you must fill out your personal information along with the address and the mode of communication.
Mention a brief description of your complaint and the other necessary details.
Once the form is complete, please review it and submit it, after which the airline's agent will contact you within 24 hours.

How to file a complaint against an international airline?

The passengers can file a complaint against any international airline by following simple steps. If they file a complaint to an airlines about the airline in which they have faced issues with the U.S. department of transportation, they are required to get onto the official website and fill out their complaint form.

File a Consumer Complaint with the U.S. department of transportation

Once you get onto the website of the U.S. department of transportation, click on the option mentioning 'File a Consumer Complaint.'
You will get redirected to the complaint form, in which you must fill out details like filing the complaint as a passenger/relative of a passenger/lawyer or travel agent.
Other personal details, name of the airline, flight date, description of your issue, and further information marked as (*).
Once all the details are complete, submit the form, and they will get back to you with the appropriate solution.

File complain over Phone Call

The passengers can contact the transportation department's representative over the phone number 202-366-2220 and request them to take action against your complaint.

File a complaint in Writing

The passengers can also write to the U.S. transportation department at the address mentioned below.

Office of Aviation Consumer Protection

U.S. Department of Transportation

1200 New Jersey Avenue, SE

Washington, DC 20590

Which airline has the most customer complaints?

Few airlines face many complaints from passengers, which is explainable as they are having a hard time. A few are listed here who face issues.

Spirit Airlines- The airline ranks first, facing the most complaints from passengers. The airline receives almost 637 complaints from customers as per the current DOT data. Half of those issues are because of flight problems that include cancellation, delay, or any deviation from the planned schedule.
American Airlines- The airline has registered almost 492 complaints in a single month about flight problems. The other issue regarding which they have faced a lot of complaints is about the refund of a flight fare, which also considers that they have provided misleading information about the fare rules to the passengers.
JetBlue Airways- JetBlue has registered over 481 complaints in a month, out f which 281 is in the category of flight problems, over 85 are about a refund, and 34 and 24 are about reservation and baggage, respectively.
Frontier Airlines- Frontier has also registered almost 383 total complaints for the month related to flight issues, refunds, reservation/boarding-related issues, and baggage issues. The highest number of complaints with Frontier Airlines is about overbooking.
United Airlines- The problem with United Airlines is more spread out than some of the other airlines. The estimated number of complain to an airline is 326, out of which 92 is about flight issue, 69 is for refund query, 35 are for reservation, and 34 is for baggage claim.