Russian Empire
Though the Empire was only officially proclaimed by Tsar Peter I following the Treaty of Nystad (1721), some historians would argue that it was truly born either when Ivan III of Russia conquered Veliky Novgorod in 1478, or when Ivan the Terrible conquered the Khanate of Kazan in 1552. According to another point of view, the term "Tsardom", which was used after the coronation of Ivan IV in 1547, was already a contemporary Russian word for empire.
Grand Duchy of Moscow
The reign of the Tsars started officially with Ivan the Terrible, the first monarch to be crowned Tsar of Russia, but in practice it started with Ivan III, who completed the centralization of the state (traditionally known as "the gathering of the Russian lands").