The main difficulty about answering this question is that there is a big difference between those cases where the number of victims is definitely proven, and those where the numbers can only be guessed at. If we are looking at known number of victims, then the most 'successful' (if you know what I mean) serial killer to date is the British doctor Harold Shipman, who is known to have murdered 218 of his own patients, mostly elderly women whom he killed by poisoning them. This went on for years before he was caught, and the real figure may have been higher – possibly 450.
Where exact figures don’t exist, of course it’s much harder to find out the truth. There have been a number of murderers in history who were certainly what we would call serial killers, but historians don’t always agree on numbers. But it’s likely that Gilles de Rais, a medieval French aristocrat and supporter of Joan of Arc, was one of the worst – a number of bodies, mostly of children, were found at his castle and he was suspected of killing as many as 600 people (he is know to have killed 200.) With the 16th century Hungarian Elizabeth Bathory, the number may have been even higher – as many as 650. The only other solo killer who matches this is the US murderer Henry Lee Lucas, suspected of up to 600 victims.
Where exact figures don’t exist, of course it’s much harder to find out the truth. There have been a number of murderers in history who were certainly what we would call serial killers, but historians don’t always agree on numbers. But it’s likely that Gilles de Rais, a medieval French aristocrat and supporter of Joan of Arc, was one of the worst – a number of bodies, mostly of children, were found at his castle and he was suspected of killing as many as 600 people (he is know to have killed 200.) With the 16th century Hungarian Elizabeth Bathory, the number may have been even higher – as many as 650. The only other solo killer who matches this is the US murderer Henry Lee Lucas, suspected of up to 600 victims.