So far, it could be any country. However, Japan based Leadership Offers percent of the Philippines, Indonesia, the massive expansion of Bitcoin. Bitcoin us/europe countries use less.
bit coin
BTC mostly use in Us and European countries. but now
BTC expand in asian countries like korea, Philippines and many more
I'm not sure if they have the biggest volume of transactions. But I know they are one of the countries that are using bitcoin. But as I say, there's no final statistics on what country are doing the most trade in bitcoin. I only know on what coin is having the highest volume that is happening on a daily basis, exchange does have that record.