re: In your opinion, who was the greatest leader in history?

The most revolutionary figure, leader wise, in history was Napoleon Bonaparte. He literally marks a bright line in military history like no other individual can.

Perhaps someone like Washington can be argued because of his unique position as the U.S. first great military leader and first great civilian leader.

Others have big questions as to whether or not the changes to society and history were permanent or fleeting. Xerxes was a study in contrasts, Alexander died so young, Hannibal couldn't translate battlefield victories into long-term success, Julius Caesar presided over the end of the Roman Republic, Frederick was overshadowed by Napoleon a generation later, Lincoln had a lot of blood on his hands (of course, so did Napoleon), Grant was deeply flawed, personally, and ultimately corrupt as President, Stalin/Hitler/Mao were literally mass murderers.

ETA: Should add Elizabeth I, here. Her defensive nature makes her appear less effective than some on here. Perhaps her overall success and longevity should score her more points than they do.