By far, the best chocolate croissant I've eaten in the Boston area is from Brit's Bakery. Their pâté feuilletée could compete in Paris, whereas even Iggy's or LA Burdick's are barely edible, and only Trader Joe's frozen chocolate croissants approach the real thing.
Glad to help. A lot of their frozen section of surprisingly good. Don't even try their pre-baked chocolate croissants, though, it's the usual disgusting stuff full of preservatives. Their fresh-baked bread is above-average for the States, but still short of par for me.