Flying is expensive, so it's not very surprising that lots of people are on the lookout for ways to rack up free flights. Perhaps the fastest way to earn free flights is to use an airline rewards card. Here are some tips about the best airline credit cards and who should consider getting one.

They're not for everybody
Airline credit cards are a great choice for some people, but they're not for everybody. First of all, airline credit cards -- and rewards cards more generally -- tend to have higher interest rates than other credit cards that offer sparser rewards.

As a result, if you're likely to carry a balance (which you should try to avoid doing if at all possible), an airline credit card is probably not for you. Whatever points and other "free" perks you might get would likely be eaten up by high interest payments.

Second, airline credit cards tend to have significant annual fees: $95/year is pretty common. (Many credit card companies will waive the fee for the first year, though.) Unless you are getting a lot of benefit from the card's perks or spending tens of thousands of dollars annually on the card, it may not be worthwhile to pay this annual fee. There are plenty of good no-fee rewards cards.

Third, a credit card tied to a single airline probably doesn't make sense unless you travel a lot with that airline. Bearing all of these caveats in mind, let's take a look at some of the best airline credit cards.

Best airline credit card for legacy carriers
The legacy carriers have very similar rewards cards. However, United Continental (NYSE:UAL) has an attractive Visa card offered through JPMorgan Chase with some valuable perks -- if you use them.

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The MileagePlus Explorer card has some great perks. Photo: United Airlines

Like most other airline rewards cards, the United MileagePlus Explorer card provides two miles for every dollar spent with the airline and one mile per dollar on other spending. The annual fee is $95.

The card comes with some great perks. The cardholder and one companion can check one bag for free on every flight. Since that bag would otherwise cost $25, this represents a significant opportunity for savings. Cardholders also get priority boarding and two free United Club passes per year. The card has no foreign transaction fees, which is important if you ever travel abroad.

United Continental has hubs in many of the largest U.S. metro areas, including New York, Chicago, Los Angeles, Houston, Washington, D.C., and San Francisco. This means that it is a convenient airline to fly for many people. That said, United frequently comes in near the bottom of customer satisfaction surveys, so some fliers may be hesitant to make it their primary airline.

Delta Air Lines' (NYSE:DAL) SkyMiles credit card (offered by American Express) is a good alternative for people looking to fly a better-rated airline. Delta routinely places near the top of airline reliability and customer satisfaction surveys. It also carries a $95 annual fee.

Like the United MileagePlus card, the Delta SkyMiles card features priority boarding and no foreign transaction fees. Moreover, its free checked bag benefit is good for up to eight people traveling in the same reservation. That makes it perhaps the best airline credit card for people with big families.

The big drawback for this card is that Delta Air Lines' SkyMiles program is notoriously stingy in terms of the value of each mile. It also doesn't come with any free lounge access. (However, discounts are available for cardholders.)

The best low-cost carrier card
If you're mainly looking to travel around the U.S. and some nearby international destinations, the best airline credit card for you might be a low-cost carrier's card. Southwest Airlines (NYSE:LUV) has a very good card: the Rapid Rewards credit card, also from Chase.

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Southwest's Rapid Rewards card is a good choice for many travelers. Image source: Southwest Airlines.

The Rapid Rewards Plus Card has a $69 annual fee. In addition to getting two points per dollar spent through Southwest Airlines and one point per dollar spent elsewhere, cardholders also get 3,000 bonus points each year on their cardmember anniversary.

However, for frequent Southwest customers, the best airline credit card might be the $99/year Rapid Rewards Premier Card. This card has a 6,000-point anniversary bonus, and the extra 3,000 points are worth more than the additional $30 for the annual fee. The Rapid Rewards Premier Card has zero foreign transaction fees (compared to 3% for the Plus Card) and you can also rack up tier-qualifying points to reach A-List status, which includes a variety of benefits.

Southwest's broad U.S. route network and straightforward pricing scheme -- you can essentially use points like cash -- makes the Rapid Rewards credit card a solid choice for many people. The biggest downside is that there aren't as many perks available as for legacy carriers like United and Delta.

Before you get too excited about picking the best airline credit card for you and starting to rack up points for free travel, make sure that you will use the card enough -- and take advantage of its perks -- to justify the annual fee. But for people who spend a lot on credit cards and frequently pay for checked bags, an airline credit card can be a savvy choice.

Adam Levine-Weinberg owns shares of United Continental Holdings, and is long January 2017 $40 calls on Delta Air Lines. The Motley Fool owns and recommends Visa, and recommends American Express. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.