Here in HKG where I am based, my favorite place for croissants (I alternate between Almond and Pain au Chocolat) is Le Salon de Te de Joel Robuchon at the Landmark in Central on the 4th floor.

In Shanghai (few experiences) the Paul at the Portman was nice (no longer there I believe).

In Beijing my favorite has been at the Sofitel Wanda Hotel at their breakfast buffet - they keep bringing fresh ones!

My experiences in mainland China, to be honest, have mostly or all been limited to 5* hotels and not patisseries/bakeries that have been set up.