Southwest Airlines, Singapore Airlines, and Marriott International are the only travel brands that have consistently appeared on Fortune’s list of the 50 most admired companies for the past six years.

Every year Fortune surveys 692 of the largest and most lucrative companies from 30 countries to create 57 industry lists and the overall top 50 list.

One explanation for the three brands’ prolific presence is that respondents select the companies they admire most from a list of companies that ranked in the top 25 percent in last year’s survey and those that finished in the top 20 percent of their industry.

Therefore, brands that appear at the top of the list once are the most likely to appear at the top again. For example, Apple, Amazon, and Google have been ranked in the top three for the past three years.

In 2014, Delta Air Lines joined the top 50 list at number 48. Southwest came in at number 9, Singapore at 18, and Marriott at 39.

The airlines are ranked very differently on the industry-specific list, which only industry experts vote on. Delta ranks first and Southwest ranks seventh on that list.

The information provided by Fortune on each Southwest and Singapore are below:

Southwest Airlines: “During a time when the “friendly skies” aren’t so friendly anymore, Southwest has won favor with customers for perks — like a free checked bag — that have disappeared from most airlines. The company has experienced 40 consecutive years of profitability, and net incomes hit $805 million in 2013. The American travel favorite is now looking to take its service internationally. Southwest will begin sending passengers to Aruba, the Bahamas and Jamaica in July 2014.”

Singapore Airlines: “With its roster of separately branded travel networks – Silk Air for regional flights, Tiger for low-cost trips, and Scoot for long-haul travel – Singapore Airlines has cashed in on the expansion of Asia’s urbanization and its growing middle class that wishes to travel beyond the continent’s major metropolitan hubs. In February its shares posted their biggest daily gain in two-and-a-half years and it’s reportedly in talks to purchase 40 wide-body jets.”

Photo Credit: Southwest jet readies for landing at San Diego International Airport. JerandSar Gimbel / Flickr