Factors we consider:

• Presence of a train station with TGV service (very fast trains) 
• Proximity to an international airport* with relatively robust schedules

*The Charles DeGaulle airport in Paris has the most international connections, by far.  Connecting flights can then get you to the city of your choice.   Many US travellers prefer to fly into CDG and then transfer to a TGV train, if their destination is served by a TGV train, the travel times can be about the same as a connecting flight.    Short hop airlines (Ryanair, EasyJet, etc) are an alternative depending on where you want to go.  If travel to the US is your main objective, as of this writing, the only direct flights into France from the US go to CDG, Orly (fewer flights) Nice (only in the warmer months).  For destinations to the SW of France, it can be possible to arrive in Barcelona and take the train north.   Please note, this information is highly subject to change.