As of 2022, South Korea has the lowest total fertility rate globally. This effectively means it has the "worst" birth rate in the world, if "worst" is interpreted as "lowest."
According to available data, South Korea's total fertility rate was just 0.78 in 2022. This is significantly below the replacement rate of 2.1 births per woman, which is needed to maintain a stable population size.
Therefore, based on the provided reference, the answer is:
South Korea.
