Best way to get from Paris CDG airport Terminal 2A to Gare Montparnasse train station

Take a cab for 58 Euro for up to 4 people; use only the official cab line.

Or take the RER B for 10.30 to Denfert Rochereau and then the 6 metro to Montparnasse -- as I recall it is a long walk within that metro. complex to the train platforms but they are pretty well signed with pictures of main line trains.

Be pickpocket proof.

The RER B + metro 6 route is only viable if you can comfortably carry your luggage up and down flights of stairs. There are many stairs at the RER B stop to get to metro line 6, and a couple more at Montparnasse metro stop. Anything heavier that 25 lb or so, and you'll wish you had taken a cab.

Balso is absolutely right about that. We have done this route getting to Edgar Quinet which is on the route to Montparnasse and it means lugging bags up and down lots of stairs. We could do it 15 years ago -- at our age now, we would not consider it. So it depends on your strength and how heavy those bags are.