Well, the epithet "the Great" isn't given for being nice. For example, Timur the Great slaughtered hundreds of thousands of people and destroyed entire cities.

Anyway, Peter the Great transformed and expanded Russia from a traditionalist (not to say "backward"), essentially medieval state on the fringes of Europe, into one of Europe principal great powers. He defeated Sweden (at that time one of the most powerful states in Europe) which gave Russia important land on the Baltic Sea, on the coast of which he built a new capital. He reformed or modernized almost every aspect of Russian society, from religion, to dress code, education, the military, civil administration etc. No other tsar of Russia has done more than Peter.