American and Delta were the world's largest airlines in 2019

American Airlines and Delta Air Lines were the world's largest airlines in 2019, as the U.S. carriers and their peer United Airlines continue a neck-and-neck challenge for the top spots.

American carried the most passenger traffic, measured my revenue passenger miles (RPM), followed by United and Delta, their respective 2019 results show. American flew 241 billion RPMs, United 239 billion, and Delta nearly 238 billion.

At the same time, Delta generated the most revenue — overtaking American by more than $1 billion — followed by American and United. Delta generated $47 billion in operating revenue in 2019, American $45.8 billion and United $43.3 billion.

While the revenue and RPM metrics are widely accepted industry barometers, other data points can also be used to rank airlines. Other metrics include overall fleet size (American is No. 1) and total number of passengers carried (also American).

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No matter what the metric, these rankings are little more than bragging rights. Few passengers book — or investors buy — based on what airline is largest.

American has led the global pack annually for several years. While the quarterly rankings change fairly regularly, and the Fort Worth, Texas-based carrier only ceded the top revenue generator title to Delta this year.

"[Given our] solid year-over-year improvement, especially given the tough environment we experienced, we know we can perform better," American CEO Doug Parker said during a quarterly earnings call Thursday.

The Boeing 737 MAX grounding and operational issues were among headwinds faced by the Fort Worth-based carrier in 2019.

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Related: Delta says passengers are spending big on its premium products

Looking ahead at 2020, the dual rankings for largest carrier remain up for grabs. Both American and Delta forecast an up to 2% year-over-year increase in total unit revenues — or how much they earn per mile flown —potentially giving the former a chance to recoup lost ground this year. However, numerous unknown variables from labor contract negotiations are both carriers to the uncertain return of the MAX could impact these forecasts.

United is giving American a run for its money on the traffic front. Chicago-based United has said it plans to grow capacity by 4-6% year-over-year in 2020, while American aims for slightly more moderate 4-5% capacity growth. Both targets, however, will be impacted by the return of the MAX and each carrier's other fleet plans.

Not including the MAX, American is due to add 31 aircraft and United roughly 26 aircraft to their respective mainline fleets in 2020. However, the former is also sticking with plans to retire its fleet of 20 Embraer E190s by the end-of-summer and 11 Boeing 767-300ERs by year-end — effectively eliminating any mainline fleet growth without the MAX.

Delta plans to grow capacity by as much as 4% year-over-year in 2020.

Related: United Airlines plans more than 700 flights from Denver hub