The top two countries hardest hit were Russia and France. They both had devastating military losses, economic hardship, and decreased morale after World War I. Russia had the most casualties in the war (roughly 3-3.7 million total deaths, including civilian estimates), which, considering their early withdrawal in 1917, makes that number even more severe. Based on casualties, Russia suffersed the most as a result of World War I.
World War I puts too much strain on Russia at the least possible opportune time. With the extra pressure on the economy and the growing dissatisfaction among the poor, Russia is ripe for revolution and the Bolsheviks seize power. Russia will no longer be the same--literally--because after that, the country known as Russia ceases to exit. The USSR is born.