"Bob Hope"?
Jesus Christ, Rob. He wasn't even well known outside the US for most of the century.
Its hard to pick one person. Queen Victoria of England was undoubtedly Person of the Century in the 1900s.
Lenin undoubtedly affect a large part of the world, from 1917 to 1989. Hitler's influence really only lasted 10 years or so. Stalin and Mao were only influential on the world stage for 30-40 years. FDR was only 12 years, and Gorbachev only 4 years.
I'm tempted to say John Maynard Keynes. This is the man who invented macroeconomics.
Let me think about this.....