It's been a rough few months for Southwest Airlines and WalletHub's research backs that up. The low-cost carrier earned a dismal score of 23.39, which is nearly half that of the 10th-ranked airline. Southwest ranked seventh in terms of affordability and second to last in terms of reliability, which considered airlines' rates of cancelations, delays, mishandled luggage and denied boardings.

Despite being the second-best U.S. airline for budget travelers and the third-safest, based on WalletHub's research, Frontier Airlines is held back due to its poor performance for comfort and reliability. The ultra-low-cost carrier ranked last in the latter category, producing the highest rate of cancelations, delays, mishandled luggage and denied boardings.

While American Airlines doesn't rank worst among the Big Four carriers, the airline clearly has some work to do to catch up to its rivals. American ranked eighth for safety, 10th for affordability, fifth for comfort and eighth in terms of reliability.?

American Airlines Group subsidiary and regional carrier Envoy Air is the safest airline but ranks on the opposite end of the spectrum for cost and third from last in terms of reliability.?

JetBlue ranks seventh overall but is the nation's most comfortable airline, WalletHub found. The category considers the in-flight experience and whether an airline offers free amenities such as Wi-Fi, extra legroom, and complimentary snacks and beverages.

Hawaiian Airlines continues to be one of the most reliable U.S. carriers, ranking fourth when it comes to the rate of cancelations, delays, mishandled luggage and denied boardings. The airline also ranks fifth in terms of affordability at 11.89 cents per mile.?

Alaska Airlines is tied with two other carriers for the title of the nation's pet friendliest and ranks third when it comes to comfort. However, the airline ranks middle-of-the-pack in other important categories such as safety, affordability and reliability.?

United Airlines fares extremely well in terms of reliability, where it ranks second. Nonetheless, the airline is held back by disappointing scores for comfort, affordability and safety.?

North America's largest regional airline ranks a surprising third in WalletHub's study, trailing only Delta Air Lines and United in terms of reliability and only Spirit and Frontier when it comes to affordability. What's more, the Utah-based carrier is tied for the most pet-friendly airline in the U.S. this year.?

Spirit Airlines narrowly missed out on the top spot in WalletHub's rankings. The ultra-low-cost carrier, which is in the process of being acquired by JetBlue, ranked toward the bottom in terms of comfort and just fifth for reliability. Nonetheless, Spirit was propelled by its affordability, ranking first at a cost of 5.45 cents per mile.

In addition to ranking number one overall, Delta Air Lines was found to be the most reliable of the 11 airlines analyzed. It also tied with Alaska and SkyWest as the best airline for travelers with pets. The airline didn't win by a landslide though. Delta's overall WalletHub score of 66.79 is just 0.22 better than Spirit.?