Nikita Khrushchev.
Khrushchev replaced Stalin in 1958, five years after Stalin died, during which he fought for power with Georgy Malenkov.
Khrushchev was responsible for what we call the "De-Stalinization of the USSR". This referred to the actions of removing the cult that surrounded Stalin, his political system, and the Gulag system. All these were in effort to get rid of the "cult of personality" Stalin had set in place, which was simply propaganda marking him as an all-powerful super-leader of sorts.
Khrushchev was also the first to place emphasis on the Soviet space program, and set forth some reforms of the Soviet domestic policy. Khrushchev was also behind the wheel of the USSR in some notable crises, such as the Cuban Missile Crisis in 1962 and the U-2 Incident in 1960.
