Updated: Nov 20, 2020
As in competitive or recreational?
Going by a percentage as some countries have larger populations and some much smaller)
If it is competitive, there are quite a number of Danish, Chinese, Indonesians, Malaysians, Singaporeans, Thai and others. These countries have badminton as a popular sport and hold quite a number of local and international tournaments a year. It is very hard to say which has the most as badminton is popular in some states/cities and unpopular in others. However there is a huge badminton community in at least 1 cities of the above mentioned countries.
If it is recreational, we would have to look at availability of badminton courts in the country or city. I think it would be safe to say that, after travelling around these countries, Singapore has the highest percentage of recreational players. This is due to how accessible and convenient it is to find and book a badminton court to play. Every neighbourhood has 1 centre for badminton and other activities. There are also stadiums and clubs where tournaments also take place. Meanwhile I had to drive for 30 minutes to 1 hour to reach the nearest stadium to play. They hardly have smaller centres like Singapore to play with convenience.
To sum up what I have said, it is very hard to say which country plays competitive badminton the most but I can confidently say Singapore easily has the highest percentage of recreational players among the population.