- Donquixote Doflamingo
- Shogun Orochi Kurozumi
- Kaido of the Hundred Beasts
- Rob Lucci
- Arlong
- Big Mom
- Crocodile
- Enel
- Wapol
Donquixote Doflamingo was so infuriated with the Tenryuubito and the world he was forced down onto, he made it his life's goal to ensure the destruction of everything the Tenryuubito held. Meaning, had he not been stopped, the lives of every innocent person in every World Gov. country he came upon, were in danger. They were conditions in Doflamingo's goal that had to be met. Beyond that, his actions in Dressrosa precipitated war across the world—so much so hearing of his defeat alone ended wars—and the enslavement of those in Dressrosa (pirate or otherwise) is some of the most disgusting actions seen in One Piece.
Shogun Orochi Kurozumi is just Donquixote Doflamingo but lesser. He holds a similar grudge against the nation of Wano and all its populace and wants to do the same thing, he's corrupted them or taken away their hope. In some cases, both. Turned a very cultured and vibrant landscape into a living Hell where their guests in the Beast Pirates can do whatever they please. Again, just like Doflamingo but to a much smaller extent in both goals and actions.
Kaido / Rob Lucci come in 3rd/4th because they have no powerful, driving ideology or goal beyond seemingly loving violence for violence's sake. Kaido has an entire crew that sells weapons of war, enslaves and murders the locals of Wano and any other land unlucky to be their territory. Rob Lucci is just a bloodthirsty person. What set's them high is their willingness to murder and that murder is actually their goal. Kaido wants a war to ruin the world, Rob Lucci is among the top assassins for the World Government because he wants to harm others. This willingness to kill without reason is what sets them higher.
Arlong might be controversial. He was a horrible person, racist and violent to humans. He wasn't mindless in his discrimination nor was he entirely a monster who gave into destruction. He could be patient, work with some humans like the Cat themed Marine Captain who he bribed. There was also the fact he didn't harm fellow Fishman and that his racism and hatred, as bad as it is to say, isn't baseless and comes from a real place. Fisher Tiger's death and the treatment of Fishman for hundreds of years helped mold Arlong. It's still his personal failing that he can't see beyond it, but he isn't Hody Jones.
Big Mom destroys entire countries for being unable to provide her with her sweet tax. She steals life spans to fuel her creations and doesn't allow anyone to leave. She betrays her supposed allies, has less self control than a ten year old and leads directly to the demise of her kin on several occasions. She's a dictator who really doesn't love anyone but herself and the lie she believes to be Mother Caramel.
Crocodile, as horrible as he was in Alabasta, isn't really the monster I think those above him are. He doesn't intend to pillage the entire world, so far as we've seen, has specific enemies that he wants to be rid of. Alabasta was a casualty in his plans, but it's not like he intends to go to other countries to continue this bloodshed. He's willing to kill, if he must, but his ambition nor his hobbies do not include the murder of random peoples.
Enel is a lot like Crocodile, he's haughty and willing to murder. But after Skypeia, he was going to go away... So, yeah, horrible human being, but his potential to hurt people (and willingness) wasn't nearly as high. Wapol was just greedy to an extreme and a lesser Big Mom.