Oui Oui Boulangerie!
"The best croissant in Paris is simply a good one within sleep-walking distance of where you happen to be staying" according to Paris by Mouth writer Eric Kayser. He was in search of the best croissant at this time.
One of the most popular is Poilane which offers several locations in Paris. It's Ina Garten's, (Barefoot Contessa) favorite in Paris.