Best options from CDG airport to Montmartre

A taxi is a fixed price of 55 EUR to right-bank locations in Paris.

If you would prefer to take mass transit, tell us what metro station is near your accommodations for more specific information but generally the idea is that you would take the RER B to into central Paris and then change at one of the central Paris stations that the RER B services to a metro line that will take you near your hotel. For example, if you are staying near Abbesses metro station, you would take the RER B to Gare du Nord and change there for Line 2 to the Abbesses metro station. The station for line 2 is La Chapelle and is above ground and Gare du Nord is below ground so you would have a bit of a walk to change trains but it is totally doable if you can manage your luggage. CityMapper and www.ratp.fr offer excellent tools for mapping out your trip if you want to take mass transit. Montmartre is hilly so door to door service with a taxi might be a good idea and also the RER costs 11.40€ per adult so depending on the number of adults that make up "we" a taxi might be cost-effective.

Note that while the RER B will always have elevators and escalors, when you transfer to the metro you will have stairs at some point in most cases. Abesses has an elevator from the tracks level and that may go all the way up. but if you have to transfer from line 2 to another line there will be stairs and most stops will have steep exit stairs. If there are several of you, a taxi is sure easier. Train will be 11.45 each; taxi is 55 for 4.

I tend to lean towards taxis from CDG in general, and it probably makes the most sense for Montmartre.
If you are near metro line 12, you would need 2 transfers, and if you are near line 2, the transfer at Gare du Nord is about a 10-min walk. If you are near line 4, it is a bit better, but in any case, even a short walk from the metro could end up very steep.