Montparnasse to CDG
 

Other than a taxi, is the Air France bus #4 the best way to go?

it completely depends where you are. If you are very close to Port Royal or Denfert Rochereau RER stops, it would be a lot easier to just take the RER. Because the AF bus stops right near the Montparnasse train station so if you weren't there, it may not be so convenient.

Assuming you want to travel from Montparnasse to CDG, the AF Bus #4 is 17€ and takes about one hour. Alternatively, M6 to Denfert-Rochereau connecting to RER B and CDG is 9.25€.

What does <i>best</i> mean: fastest, cheapest, most comfortable, or some other criteria?

Arriving on train from Bordeaux AT Montparnasse.
What is "M6"?

Métro line 6.

it means metro line 6, that's all. I'd definitely take the AF bus if you are arriving at the train station and then want to go to CDG. Otherwise, you'll have to take the metro to the RER station. I think that is a pain, but to each his own, it depends on how much baggage you have.

Thank you.