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.
If we want to determine the capacity and the capability of a particular country to deal with people transaction , one way is to look at its region or size. And it is not surprising that China could be the number one county who uses nitcoin in transaction in associate n its region. But currently China don't accept bitcoin becasue there is some news that they want to establish their fist own form of crypto-currency.