Final answer:
Ivan IV, known as Ivan the Terrible, was the first Russian ruler to take the title of tsar. He expanded Russia's territory eastward and centralized power by subduing the nobility, marking the beginning of imperial expansion into Siberia.
Step-by-step explanation:
The first Russian ruler who took the title of tsar, expanded the territories of Russia to the east, and crushed the power of the nobility was Ivan IV, also known as Ivan the Terrible. He became the first to officially use the title Tsar of all the Russias and his reign marked significant geographical expansion and the imposition of absolute rule over the Russian nobility. Ivan IV's bloody reign included the annexation of the khanates of Kazan, Astrakhan, and Siberia, and saw the beginnings of Russian territorial expansion into Siberia aided by Cossack explorers and settlers.
Compared to other Russian rulers like Peter the Great and Catherine the Great, Ivan IV was notable for his direct and brutal approach to governance. While his contemporaries, such as Catherine the Great, continued Russia's territorial expansion much later, it was Ivan the Terrible who first declared himself Tsar and initiated significant consolidation and expansion of Russian territories.