Probably China. They are the biggest player of bitcoin for many years. I think they are also the biggest bitcoin miner as far i know. The next i think US but there are problems regarding on the legalization of bitcoin there. Maybe Japan can surpass US or even China due to massive adoption of bitcoin there. Many establishments there are now accepting bitcoin. Bitcoin growth there is phenomenal.
We could consider the country's size if we are going to know which has the most bitcoin trade. But unfortunately China recently banned bitcoin in terms of transaction so it may affect the volume of transaction in their market specifically on trading, investments or in business. But if there would be a regulation in terms of transaction , I prob chooses America as one of the country with a highest trading stock out of all countries.
China based exchanges have closed but Chinese residents holding bitcoins are still using exchanges and trading