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.
I also think so, that China is the biggest trader of Bitcoin. But I also heard some about China that it's Govt. were planning to stop bitcoin transaction because of some features of bitcoin that is unfavourable to every Govt. in every economy.