According to my knowledge it previously was China had the highest bitcoin trade volume, but as of now banning of ICO's in China doesn't keep China above in the list maybe now it is the USA or Japan.
Japan is right answer. USA is also trying to get most of the ICOs but they are not making success in it. While on the other hand if you talk about Japan then you ultimately come to know that they have most investment and trading and bitcoin shares than any other country, China was once but now they are not at number one.
It is depending on the number of bitcoin users in a country and the government behavior about bitcoin. I think in present time as Japan has accepted bitcoin as legal currency therefore i think in japan trading bitcoin will be more than any other country, but still we can expect that very soon other countries will also start using bitcoin as legal currency and hope that gradually states will continue accepting bitcoin.
I believe that china has one of the biggest contribution in the market since the electricity consumption in their country is pretty low. Bitcoin mining is a huge advantage in their place since you can earn more profit because of the low electricity bills.