Hitler
Stalin
Pol pot
Mao
My list is this way not only because of the body count, but also because of the ways and reasons.
Hilter not only killed millions, he also amde it so industrialized he could've killed millions more. In addition his expansion mania caused the deaths of many millions more around the world and the destruction of many places in europe.
Stalin killed many millions and in addition he sent quite a lot of people for "vacations" in siberia. Also, he killed his generals, causing the soviet losses in WW2 to be much higher than necessary.
Pol pot was, to my opinion, too mad to understand what he's really doing.
Mao, although responsible for the deaths of millions, caused most of them as a result of misjudgment and not becaue he wanted to kill them.
I didn't put any ancinet leaders because I think the atrocities they commited back then weren't as horrible as if they were commited tosay, as then massacaring your enemy was in a lot of ways the standard. Also, most of the atrocities of the ancinet world were as a result of wars.