I have to agree with Chowhounds in other threads who have recommended Petrossian's. After having tried croissants or pain au chocolat at Patisserie Claude's, La Bergamote, Ceci-Cela, Balthazar, Payard, Eleven Madison Park, Bouchon, and Pret a Manger (I liked theirs a good bit, actually!) and probably others, Petrossian's is the only one which crosses the threshold into good Parisian croissant territory... It tried the pain au chocolat there, and it's wonderfully flaky, it's buttery, it's got good chocolate, and it disappeared in a flash. Delicious.
The Vegetarian New Yorker: http://vegny.blogspot.com/