Alexander conquered the known world by 30 and was wise enough to allow the conquered people some autonomy and incorporated there strengths into his government. Hard to argue against him.
Genghis Khan would be my choice. The way I look at it is if I saw any of these guys at my doorstep who would I fear the most, and it would damn sure be the Mongols. And China to the Atlantic is a pretty incredible feat. What sucks though is that if Mongke Khan wouldn't have died from disintary, the Mongols could have very well stamped out Islam.
George Washington,.. he led the revolution, and was awarded the Presidency, he had all the power in his hands, then without precedence in history, he gave the power away,.. for the sake of democracy.