This church (which was built in the 1820's) might seem small and simple from the outside, but the interior is richly decorated and truly impressive. Vincent de Paul, a 16th-century Catholic priest who was later canonized, is buried here - be sure to check the beautifully ornamented reliquary.
Wikipedia (French) https://fr.wikipedia.org/wiki/Chapelle_Saint-Vincent-de-Paul_%2528Paris%2529
Address Rue de Sèvres 95, 75006 Paris
Coordinates 48°50'55.85" N 2°19'18.641" E