I don't know how you would describe "worst". Certainly there were some cases that were more difficult then others but it was never a case of 'good ghost, bad ghost'. The way it was explained to me as a child was that the world was full of echos of past lives. Like ripples on a pond. Most people swim through the ripples without noticing they exist but occasionally someone will be 'buffeted'. The bigger the ripple the stronger the turbulence and the more likely the memory of the past life would be seen. I was being simplistic when I called my father an exorcist. He's job was more to deal with the living, educating them to not be afraid of what they were experiencing. Nine times out of ten, once the fear was gone so did the 'ghosts'. They hadn't of course it was simply that the living had learnt to swim through the ripples.
Occasionally there was an echo so terrible that no one in their right mind would be able to happily live alongside it. Then the only options were to either remove the object from the home or, if that was not possible, remove those who were sensitive of the event from the scene. As 99.9999% of people won't see anything in their lifetimes it's quite reasonable to sell a house because for you it was haunted.