Any killer can be extremely strong with the right build and perk setup but if we are going based on purely power without add-ons or perks I would say Hillbilly and Spirit can do really well without any. If we take perks and addons into account Doctor, Wraith, Bubba, Michael and Ghost Face become really good. All the other killers have a bit of a steeper learning curve but can be just as powerful if not more so if you learn how to use them.
All killers are very strong if they’re being played by the right person, however I think Hillbilly or Nurse are definitely S tier.
Freddy & Nurse if you know what you doing
Ghost face 100% or pig
Trapper played right might be one of the best killers