It depends. There’s no single best flight API. Your choice depends on the specific problem you’re trying to solve (e.g. enable flight and fare search, or track flight status with departure and arrival times, or enable flight booking). Generally, there are two basic options: source data from global distribution systems (or GDSs, the major, worldwide flight aggregators) or directly from airlines. In some cases, you can check APIs by tech providers like FlightStats.
If you need the widest airline coverage and you want to implement flight booking, check GDS APIs by Sabre, Travelport, and Amadeus. Each of them covers about 400 active airlines. They search for flights and low fares, and do booking and ticketing. The problem with this approach is that some airlines like Lufthansa set surcharges for booking through GDSs because they want to encourage direct booking or direct cooperation with resellers.
So, the option is to integrate and partner directly with each airline you need. That, however, presents an even larger number of problems as there are only about 40 airlines that have standardized XML-based APIs and each of them is slightly different. So, the engineering effort may be enormous. On the bright side, with direct connections, you get the widest ancillary booking support, seat selection, baggage customization options, etc. The most balanced approach to flight search and booking is to combine GDSs with some direct integrations.
If your goal is general info without booking capabilities, you may not need GDS or direct integration. The first place to go for fresh flight fare data is ATPCO, the main fare distribution provider. The largest pool for timetables, routes, and connections is provided by Innovata, a travel tech company. Also check FlightStats and Flightradar24 for flight and airport details like delay indexes, arrivals and departures, aircraft equipment, airport FIDS, flight status, etc.
If you need something simple and don’t want to go through raw airline data, you may contact OTAs or metasearch platforms to integrate their APIs. The key provider here is Skyscanner, but also consider Expedia or Kiwi.
For more details, have a look at our travel API’s articles.