One of the most famous of Parisian foods after the croissant is surely “La Crêpe”. On every third street-corner throughout the city there is a small stand where pancakes are available. Often there are only 3-4 basic kinds, and usually they are not as good as you expected. It might not even be prepared in front of you, but simply reheated. It’s a pity to eat a crêpe just to satisfy hunger rather than really enjoying it.
There are popular and without doubt good Brittany crêpe-spots near Montparnasse (such as the Crêperie de Pont-Aven, or Saint-Malo, 54 and 53 Rue du Montparnasse), but they are all too well-known.
What I recommend you is not the big and famous kind of place.
Le Pot O’Lait is just a small crêperie, not especially successful with a wannabe authentic interior, but it is truly an excellent place to eat pancakes, as is evident from the number of people inside. It is dedicated to crêpes and galettes only. A galette is a large buckwheat flour savoury pancake, while crêpes are made with wheat flour and usually have a sweet filling.
There are around 20 savoury salty kinds and at least as many sweet ones. Sweet can also be flam béed and served with ice-cream or whipped cream. ‘Squirrel’ flavour (écureuil) contains sweet nuts and great savoury ones are Capucin or Fermière. These are personal choices, but I’m sure the others are just as good.
One more thing that makes this place special is the warm and enthusiastic staff.