What country has the highest female to male ratio?

According to the Central Intelligence Agency World Factbook, as of 2013 the country with the highest female-to-male ratio is Estonia. Across the entire population, Estonia has only 0.86 males per female, or about 1.16 females for every male.

Estonia has a much higher female-to-male ratio than the world average of about 0.98 females per male. Notably, Estonia's sex-at-birth rate of about 0.94 females per male is very similar to the world average of about 0.93. Across the entire population, there are a total of 139 countries that have more females than males, and only 91 that have more males than females.