Andrei Chikatilo gets my vote for the worst serial killer ever. There is an excellent book about him- Camrade Chikatilo The Psychopathology of Russia's Notorious Serial Killer by Mikhail Krivich and Ol'gert Ol'gin. This man was just sick!
I also think that the Green River Killer is perhaps one of the worst- after all , it took how many years to catch him? Dennis Nilsen was a pretty inventive serial killer, cutting up his victims and flushing them down the toilet. Ted Bundy started off as a pretty smart killer, but got pretty sloppy as he escalated. Gacy was stupid for keeping the bodies in his house, he should have kept dumping them in the water, how did he live in that house? THE SMELL!! Dahmer- what can I say? One of the worst most deffinately, drugging people, drilling holes in their heads and pouring acid in the wound, keeping flesh in the freezer and a skeleton in his shower-insane!
There are soooooo many notorious killers. How sad!