If we speak in terms of numbers that would be Dr. Harold Shipman of the UK with 218 proven victims and a probable 250+ He killed his patients mostly old women with diamorphine injections.
Hitler
Michael Myers. If by best, you mean worst, see the links below.
Easily the world's worst Earthquake.
the worlds worst flood was in poland 1997 :)
Lightning is called the THUNDERSTORM'S WORST KILLER
Tornadoes are called a thunderstorm's worst killer.
One of the worst in American history. Before his execution, he confessed to at least 33 murders of young men and boys.
Tornadoes are called a thunderstorm's worst killer.
No country is bad but the man who said Antarctica is is the worst
Teke rerri
Wikipedia serial killers by country. This lists both sexes so you will have to pick out the females as needed. ANSWER: Wilkipedia's list of female serial killers is woefully out of date. There are well over 600 known cases. The best (by far) introduction to the "worst" female serial killers is: The World's Creepiest Female Serial Killers. (online)
attack of the killer tomatoes