Vietnam (mask wearing), literally one country away from China, where the breakout started.
Everyone here has worn a mask, for three months now. They have been legally required now for everyone in public, for two entire months.
Deaths/million = zero.
The country is now opening back up.
But China also falls into the category of mask wearing countries.
And just 270 cases and no deaths? Well, if you can use Vietnam as an argument for the effectiveness of masks then you must also consider other factors. One of which is that the median age in Vietnam is 32.5 years. But you must also disallow other factors, such as perhaps some acquired immunity. Or maybe it's the fish soup. Or survival of the fittest from the war. Since according to this report from one living there the sanitary practices were not very good.