Mao, at least in terms of body count, wins hands down, with some estimates as high as 78 million killed either directly, or through mismanagement.
Stalin is next, body count in the 20-25 million range.
Hitler, (sorry, had to be said) and Leopold II of Belgium both come in around the 15 million range.
Add a string of Roman emperors into the mix, and a similar string of Chinese warlords, and lives lost to personalities in history is staggering.