The international airport (PTY) at Tocumen is located about 24 km east of Panama City. Upon arrival in Panama, one immediately notices the tropical warmth as one exits the airport.
For the fastest route via the "Corredor Sur" it takes about 30 min to get to the city center. With a taxi you should plan about 30-40 dollars, which costs more than twice as much as a trip from Panama to David with an air-conditioned coach.
For a trip in the first hours of the morning you have to be very patient, because every day from the south-western direction many people drive to work in the city and the traffic jam lasts for several hours. For a return trip to the airport, the other direction is always congested in the afternoon from 4pm. Although a new mount system was introduced in June 2015 to allow for faster processing, the congestion is still unmissable every day during peak hours.
Many well-known car rental companies offer their cars directly at the airport, including of course the global players such as SIXT, AVIS, Budget and Hertz.
If you want to save money, you can take the metro bus into the city, but the stop is not directly at the airport and you actually need a metro card.