China, Russia and possibly Korea (which army is far larger than Japan's) come to mind. While Japan still does have 100+ million people the population is going to shrink and those who live there are getting older and older. The low birth rate will make it hard for Japan to maintain a standing army, the ageing population also makes it harder to maintain a reserve force.
South Korea or a reunited Korea - keep in mind that I'm talking about possible future scenarios, decades from now. There has been and still is a lot of animosity between Korea and Japan [1], this goes back centuries.
