As far as I know, there is no leader in the bitcoin world. It is decentralized, there are no leader to control the bitcoin. How can I think or determine that Japan is the leader, what will be the basis to become the leader. To become a leader that leader must command and control an organization.
For me country that have many bitcoin users , have laws that support bitcoin , easy sell /buy mining tools , low electricity bill can have real effect in bitcoin price so it will be leader and if it ban bitcoin price will really Down