I'm not sure how to approach this category. Should I take it as characters, who I think are bad as characters ( badly written like Hody Jones or utterly boring as in Buffalo). Or should I take it as characters that I love to to hate (fantastically written characters like Orochi, Caesar Clown, Big Mom or simply effective douchebags like Stelly, Flampe, Saint Charlos)
Well, since this is an event to celebrate a decade of One Piece, I'll use the latter reasoning. So these will be characters, that I actually think are great or even amazing characters. Characters that I love to hate. I'll be applying this same logic to the least favorite moments as well. It's harder to apply to least favorite fight and arcs, but I'll try that too.
Orochi
Oh damn how detestable this obnoxious guy is is. A true slimy rat. A brilliant villain - and actually a really formidable one! He has a fruit with a lot of potential fighting power. Yet everyone makes fun of him, he still takes the threat of the "prophecy" seriously. And that is what has given him the edge this far! As we know, he seems to be one-step a head at every turn. Liked how the last flashback gave us insight into his nature. He is a very greedy man by nature, but he is also a boy from poor conditions. He was fast to adopt the plan of revenge to acquire a lifetime of luxury for himself. But at the same time he was baffled by the thought, that "someone like him" could actually achieve something big in life. It was actually rather humane about him. So he is an evil sleazy bastard, but his humble origins also make him a cautious man just fervently protecting what "just someone low like him" has achieved by working hard. I think he is a really good villain character.
Big Mom
I have such conflicted feelings about her. She is definitely evil. The things she does are not in any way justifiable. Even if by surface, her actions are pretty good. She cares for her family, she cares for the diversity of life and has a reliable passion about sweets. But all for selfish reasons. She cares for her family, so she can have people to use for her needs and to increase her own power. She cares for diversity, just for the sake of surrounding herself with interesting people or beings. She loves the admiration and love she gets from her subordinates and family, but doesn't really give true love back. She doesn't respect the freedom of the people she "loves", because then they are not anymore "hers" if they are free. Even her passion for sweets is eventually nothing else but hedonistic gluttony. Yet knowing her background, you can't really just out right hate her. She is innocent by nature. Someone stuck into a mental state of child that goes thorough a rough child hood. When she loses her memory, she actually becomes a cheerful, helpful and insanely powerful child. Overall - an extremely great character!
Donquixote Doflamingo
He is extremely cruel, calculative and manipulative. Yet, there is also something absolutely charismatic in his cocky overly confident nature. Or is it over confident even? His power and skill are no doubt damn admirable. He also genuinely cares for (at least some) of his family. But still at times seem to be completely unable to show empathy to anyone who isn't approved by him. He runs his mafia-like business like a true boss and a badass. Definetly another 5/5 villain.
Special mention to Van Der Decken, who is kinda best of both worlds. Genuinely kinda badly written, lame and forgettable villain. But also absolutely disgusting harasser and stalker of teenage mermaid princesses.