Just a few years ago, having Wi-Fi service on an airplane was a rare luxury. Low-cost carriers like AirTran and Virgin America were the first-movers, installing Wi-Fi on their fleets in 2009 .

Today, in-flight Wi-Fi is much more common in the U.S. Still, some airlines are much further ahead of others in this regard, and in-flight Internet services are not all created equal. So which airline has the best Wi-Fi today?

The numbers

If this were just a numbers game, there would be a clear winner. Delta Air Lines (NYSE:DAL) has Wi-Fi service installed on more than 800 aircraft, which represents its entire domestic fleet with the exception of small regional jets (those with 50 or fewer seats). Delta is also starting to roll out Wi-Fi on its international flights .

Delta has the most Wi-Fi equipped planes of any airline.

In terms of consistency, Virgin America and Southwest Airlines (NYSE:LUV) subsidiary AirTran are still the only two carriers with Wi-Fi on every flight . However, Virgin and AirTran are much smaller than the top players in the airline industry, making it easier for them to achieve that feat (and less useful for most travelers).

Satellite vs. ground

However, there are big differences in the quality of different in-flight Internet services. Most U.S. airlines with on-board Wi-Fi have contracts with market-leader Gogo. Gogo uses air-to-ground technology that sends data between an antenna on the bottom of the airplane and cell towers on the ground . This service is relatively cheap to install, but it won't work over water -- where there are no cell towers! -- and bandwidth is very limited.

The alternative is satellite-based Wi-Fi. This comes in different "flavors" based on the piece of wireless spectrum being used . While satellite systems are more difficult to set up, they can work just about anywhere in the world. Furthermore, in most cases they offer faster connection speeds. Even Gogo's CEO recognizes that satellite is the future of in-flight Internet, and the company plans to gradually migrate to satellite based offerings.

And the winner is...

Southwest Airlines has the best Wi-Fi offering in the U.S. today.

Satellite's superiority over air-to-ground systems makes it fairly easy to pick the onboard Wi-Fi winner: it's Southwest Airlines. Southwest uses a satellite based Wi-Fi system from a start-up called Row 44. It already has Wi-Fi on all of its 737-700s and 737-800s, which account for about 75% of its current fleet (and this percentage is growing over time ). Additionally, Southwest's AirTran subsidiary has Wi-Fi on all of its aircraft, but uses the Gogo air-to-ground system.

Southwest also has a price advantage over most of its competitors. Southwest charges a flat fee of $8 for a full day of onboard Wi-Fi . This means that if you have one or more connections or are making a same-day roundtrip on Southwest, $8 covers you for all of your flights.

Gogo offers a similar 24-hour access pass for the many airlines it serves, but this one costs $14. (The high price serves to deter most travelers from using it, freeing up bandwidth for the few passengers who are willing to pay that much .) However, if you are a frequent flier, you're in luck at Gogo-equipped airlines. Monthly passes cost $39.95 (for one airline) or $49.95 (for a pass that works with any airline that has Gogo service ).

Looking to the future

Southwest may have the best in-flight Wi-Fi in the country today, but it could lose its crown fairly soon. JetBlue Airways (NASDAQ:JBLU) has been hyping a new "Ka-band" satellite-based system for several years now. JetBlue claims that this new technology will offer significantly more bandwidth than rival solutions, allowing everyone on the plane to use the Internet just as if they were at home .

The FAA finally approved JetBlue's new Wi-Fi setup last month, and the company is now starting to install the new antennas on its aircraft. JetBlue expects to equip all of its Airbus planes with Wi-Fi by the end of next year , before moving on to its Embraer (NYSE:ERJ) planes in 2015.

If this Ka-band system lives up to the hype, JetBlue could be the clear winner for in-flight Internet within two years. For now, though, JetBlue is still behind the curve. Southwest's Internet offering isn't perfect, but its widespread availability, low price, and somewhat better performance than air-to-ground systems makes it the best thing available in the sky today.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.