Boulangerie in 16th?
The best croissant is always from the boulangerie closest to you... not really worth making a safari to bag "the best" there is. In your case, La Petite Marquise on the rue Boissière @ the place Victor Hugo is actually quite good... and also has a salon de thé.
If you want to walk the 500 metres to the rue des Belles Feuilles market street, the Boulangerie Lalos almost next to the Monoprix occasionally places in the top 10 in croissant contests... but takeaway only... closing day is (I think) Wednesday but doublecheck.
On the avenue Victor Hugo @ rue Longchamp, the very good boulangerie and salon de thé Huré... great terrace too !... open Tue to Sun from 6:30am. And Laidback's favourite Patisserie des Rêves is 2 minutes away... opens at 10 or so during the week, earlier at weekends.