There are many ways to get back and forth to the two main airports in Paris, Charles De Gaulle and Orly. Here is a comprehensive list of different options.
I’ve personally found a licensed Paris airport taxi is the best and most reliable way to get to and from the airport. Taxi Leader has an excellent service, offering clean and comfortable late model vans and sedans at reasonable prices starting at 55€ for a sedan going to Charles De Gaulle. Once you book online, TaxiLeader.net will send a confirmation along with the driver’s mobile number. Most drivers speak =English, which is a plus. The driver will be waiting at luggage exit holding a sign with your name on it. You don’t have to pay in advance, just have to pay in cash directly to the driver.
You can also hail a taxi from the taxi stand but sometimes there are long waits. There is no flat fee from the airport to the city, so it’s still best to reserve a taxi in advance, as the price is usually the same or sometimes more than the taxi service, depending on traffic.
Air France operates a bus service to the airports. Fares to and from Orly are 12.50€ one-way, round trip 19€, for Charles De Gaulle it’s 17.50€ one-way, 25.50€ round trip. Travel time from Orly is approximately 45-60 minutes, De Gaulle, approximately 50-70 minutes. Stops include Etoile/Champs Elysees, Montparnasse station, and Porte Maillot. Buses run every 20 to 30 minutes from 6AM to 11:40PM.
Another alternative to Charles De Gaulle is the Roissy bus leaving from/dropping off at Opera. Fare is 11€ one way and buses leave every 15 to 20 minutes. Opera to Charles De Gaulle, buses run 5:30AM- 12:30AM, Charles DeGaulle to Opera buses run 6AM to 12:30AM. Travel time is approximately 75 minutes.
There’s also a Roissy bus that runs between Orly and metro/RER station Denfert Rochereau. Cost is 7.70€ one way and buses leave every 8 to 15 minutes. Orly to Denfert Rochereau, buses run 6:00AM- 12:30AM, Denfert Rochereau to Orly buses run 5:35AM to 12:00AM. Travel time is approximately 25 to 35 minutes.
The RER B commuter train is available to and from both airports. The train stops in Paris include Chatelet, Gard du Nord, Luxembourg, Saint Michel/Notre Dame, Port Royal and Denfert Rochereau. Trains leave every 10 to 20 minutes. The fare to/from Charles De Gaulle is 10€ one-way. For the train to Orly you have to take the Orly Val shuttle train and then the RER B. Cost is 12.05€ one way. Charles De Gaulle/Orly- to Paris, trains run 4:56 AM to 11:50PM, the same schedule in reverse direction. Travel time is approximately 35 to 40 minutes.
Although the bus and trains work well, there are often delays and overcrowding, so your best bet is to hire a licensed taxi service with a clean taxi and a good chance of English-speaking driver.