Final answer:
Russia lost the most territory among the Allied powers following World War I, leading to the end of the Russian Empire and the establishment of the USSR.
Step-by-step explanation:
The Allied power that lost the most territory following World War I was Russia. After the Russian Revolution in 1917, Russia exited the war and signed the Treaty of Brest-Litovsk with the Central Powers, losing a significant amount of its western territory. This led to the end of the Russian Empire and paved the way for the establishment of the Union of Soviet Socialist Republics (USSR). Meanwhile, France and the United Kingdom gained territories, particularly from the defeated Ottoman and German empires, as a result of the Treaty of Versailles. Consequently, the Allied power that faced the largest territorial loss was indeed Russia.