The answer is - North Korea.
Apart from North Korea all three other countries experienced decline in their populations during the 2000's according to their official censuses. The fertility rate is low in each of them, and they are all in danger of population decline, but that didn't happened during the 2000's. The situation is best in North Korea with the birth rate but the population is growing very slowly because the life expectancy is relatively low, on the other hand, Japan and South Korea have very low birth rates but their population is living much more so the decline is very low, but it will be much quicker for sure in the coming years once the old population is gone. China was actually controlling the birth rate because it has huge population but still managed to make a small decrease. So all in all, North Korea has the best future from demographic perspective, Japan and South Korea are in terrible position which will be more obvious in the coming period, while China is trying to reduce its population on purpose.