definitely Lenin
no Russian Revolution or USSR without Lenin, at least not likely in the form it took
no Stalin without Lenin
no Hitler
no Mao
of course those persons could still have existed but they would not have become the notorious mass-murderers we know and loathe
there might have been other really bad things in the 20th century, but Lenin had a uniquely evil role in shaping the Russian Rev. and that led to uncountable millions of deaths in the 20th century plus so many totalitarian govts
Russia was rebelling before Lenin. They would have rebelled without him. Someone else would have stepped up - Trotsky maybe. Or Stalin. Imperial ineptness and the unpopular war would have still seen to that.