The cheapest would be either bus 350 or 351 at three tickets t+ or 5.70€ or 6€ if you purchased tickets from the bus driver.
You might also consider the RER B with any métro connection to your final destination at 10.30€.
As for passes, if your visit fits a weekly Navigo fare, valid Monday through the following Sunday, it may be of interest to purchase a Navigo Découverte card and load the weekly zone 1-5 fare at 22.80€.
Alternatively once in Paris, you can travel about on the métro, bus, tramway or funicular with the ticket t+, 1.90€ each or 10 for 14.90€.
To learn about tickets, passes and fares go to ratp.fr and click the menu icon in the upper right and select "Getting Around". Then select tickets and fares. The same site can be used for route planning and all other transport info. See parisbytrain.com to learn about public transport.
Please be advised that the Navigo Découverte kit cost 5.0€ and load the weekly zone 1-5 fare at 22.80€, hence the total will be 27.80€.
Here's a life lesson for you: in life in general, 'best' and 'cheapest' are generally mutually exclusive. Choose which one is most important to you.
Nobody in their right mind would ever assume the cheapest restaurant or hotel or shoes or phone would also be the best. Same with transport.
Thanks for the semantics lesson.
If I were king of TA, my first official act would be to order the software engineers to prohibit anyone from using the word ¨best¨ in a post.
My second would be to hire forum moderators and ban forever the use of the option ¨removed by the TA community¨.
FWIW, I found taking a taxi from CDG to my hotel the most expedient - and very reasonably priced, as the price is either €50 euro or €55 euro, depending on hotel location. If you want to save money and can afford the associate time, take public transportation. Be sure to note the potential issues (ripoffs, etc.) for each option you consider.
Thanks to those of you who have genuinely offered helpful advice related to my actual query.
If you decided to use the public transport just be vigilant of the random pickpockets.
For those of us attempting to read the minds of posters, OP, it's certainly not semantics. I would *definitely* recommend the taxi. For a whole bunch of reasons that take too long to type out. It's definitely the *best* way to travel (you get to see Paris as you enter! Look to your right - there's a hill with Sacre Coeur on it!) and it is not that expensive compared to buying RER tickets and then metro tickets.
Some of this depends on your style of travel, about which we need more information. We like to walk Paris. There are some métro stations worth seeing (Arts et Metiers, Louvre) but I'm not sure they're *that* important on a 4 day trip. Planning to walk from, say, Notre Dame/Latin Quarter/Musée Cluny area (with options for Hotel de Ville, Sainte-Chapelle and Ile-Saint-Louis nearby) past the historic cafés and then up to the Seine to see the D'Orsay (at least from outside) and Les Invalides...and then on to Eiffel Tower...it's doable. If you start early, you can do all of this in a day (but probably not go inside the D'Orsay - but you could perhaps work in Napoleon's tomb...it's pretty unusual).
But do you like art? Or history? Or investigating the history and culture of a place? I'm planning a trip with a youngster who is extremely excited about chocolate shops, Angelina's, Eiffel Tower, and the castle inside the Louvre. She thinks skulls are interesting but ultimately not her thing, but she may want to see the archaeological area under Notre Dame.
Organ concert at Saint-Sulpice? Drinks at the Hemingway Bar (and to see the ritzy interior of...the Ritz), Tuileries Gardens...the exterior of the Louvre? What do you want to see. Because if it's walkable, then the ND pass isn't necessary (you can just buy a carnet of 10 tickets if you find you want to go up to Montmartre - we often take the metro up there, and then walk back, usually staying on the Left Bank).
Main point: where are you staying? Because there are other options (Roissy Bus) but no one is going to spend a lot of time explaining more without more details from you.