Obviously Gorbachev, since he democratized the evil empire. Asking for us to exclude that answer is like asking who the best President of the 20th century was and then saying "Well if you select FDR, give a reason other than the New Deal and winning World War II"
But if you're counting raw ability to build the state and increase its power then Stalin.
Thirdly, the one who was able to somewhat humanize the state as well as preside over its heyday, Khruschev. But what he did was easy, more a function of when he came to power and what situation he inherited than his actual achievements. The break with China, Virgin lands campaign and Cuban missile crises were fiascos.
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