We will be coming from Fontainebleu. It wil be a Tuesday around 4-5pm on a national holiday. Should we drop off at Gare de Lyon, Montparnesse gare or Orly. Staying in the 6th. I am inclined to drop off at a station. Also train from Fontainebleu is not convenient as there is no rental car agency at the station and train fare for 4 is not practical.

Well, if you're staying in the 6ème, the obvious choice is Montparnasse.

Sure, Montparnasse, as long as the outfit you are renting from has an office there.

Thanks for the help. I just hope there is not too much traffic.

DOCK,
We came from Fontainbleau and dropped off the car at Orly. An easy option re traffic and the trip into Paris by bus, and then metro, was easy too - even though we didn't have any previous experience).

Or for outside Paris at EuroDisney and take RER train in from there.