Final answer:
Catherine the Great added the most territory to the Russian Empire in Europe, including parts of Ukraine, the northern Caucasus, and extending to Alaska. Her campaigns against the Ottoman Empire had a significant impact on the territorial expansion of Russia. Although Peter the Great was also a major figure in territorial expansion, Catherine's contributions were larger in scale.
Step-by-step explanation:
To determine who added more territory to the Russian Empire in Europe, it is important to review the contributions of the rulers in question. Catherine the Great is notably one of the most significant territorial expanders of Russia. During her reign, she expanded Russia to include what is now Ukraine, the northern Caucasus Mountains, and even stretched the empire's reach to Alaska. Catherine's ambitious campaigns against the Ottoman Empire led to significant gains, including fertile riverlands and control over the northern coast of the Black Sea, as she aimed to see Istanbul as an extension of Moscow's reach.
Comparatively, Peter the Great was also a major figure in expanding Russian territory, having captured the Ottoman-held territory of Azov in the Crimea along the Black Sea and modernizing the empire. However, the scale of territorial acquisition under Catherine the Great's rule, particularly in Europe, was larger.
Empress Elizabeth and Empress Anna made contributions to the Russian Empire's expansion as well, but not to the extent of Catherine or Peter. Based on historical records, Catherine the Great is the leader who added the most territory to the Russian Empire in Europe during her rule.