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.