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.
Japan in the near time, I am herein sure, will go round China and USA. Every day I read news that in Japan, both restaurants or shops the new begin to accept payment in bitcoins.