The leader that was a member of the Communist Party, but made the most changes against Communism when he took office was Mikhail Gorbachev.
Explanation:
Mikhail Gorbachev is a Russian politician who served as the last secretary-general of the Soviet Communist Party from 1988 to 1991.
His attempts at reform led to a diminution and an end to the Cold War, for which he was awarded the Nobel Peace Prize, but at the same time the Soviet Communist Party lost its former political power, eventually leading to the dissolution of the Soviet Union.
As Gorbachev came with the Glasnost, and the Perestroika he introduced major changes to reform the economy. They included the opening to the West, ceasing the total control of key sectors like the energetic oil and gas complexes that became modernized. The economy suffered the transition as many things were obsolete and not healthy, in this process the removal of communist policies in the administration of areas opened the catalog of industries for international capitals to invest in Russia.
The political reforms from Gorbachev also stimulated the creation of political opposition that practically was absent as the single party system came to an end. Human rights were also after a long time emerging as part of other international institutions coming to overwatch the transition of post-soviet Russia.
