Le Marais, Saint-Germain and Montmartre can all create a memorable Paris stay, but they suit different itineraries and different ideas of what 'central' means.
Le Marais for historic streets and evening energy
Le Marais combines central access with restaurants, cafés and a strong neighborhood atmosphere. It is useful for travelers who want to walk through the historic core and still have plenty nearby after sightseeing.
Saint-Germain for classic Left Bank Paris
Saint-Germain works well for cafés, central walks and easy access to the Seine, Latin Quarter and museum areas.
Montmartre for character and views
Montmartre feels more like a hilltop village and has a strong visual identity around Sacré-Cœur, but some central sights require longer transit.
Choose by daily routine
Compare the neighborhood with the places you expect to visit most rather than selecting an area only because it is famous.