One Piece has a lot of bad eggs. It's an anime about pirates, so it figures that Monkey D. Luffy and his Straw Hat crew would have to contend with their share of unscrupulous scallywags. The irony is that the worst villains are on the side of the law.
Enter the Celestial Dragons. These are mostly World Government officials and, thus, top the hierarchy of authority. Sadly, they use that power to crush others under their boots. The combination of rotten attitudes and undeserved positions has led to some truly awful excuses for human beings and One Piece fans just love to hate them. Even the most merciless pirate would spit on these tyrants.
5 Saint Rosward
The head of a family of three and a lifelong part of the autocratic establishment, Rosward is the further thing from a saint. He sees slavery as a natural part of life. They're just property, so mistreating them is their owners' prerogative. That rule is so ingrained in his mind that he's shocked when others diverge from it. While not exclusive to him, the embrace of such an inhumane trend makes him despicable on a fundamental level. One trait, however, helps Rosward avoid the bottom tier.
To his credit, he seems to love his children. When Luffy punches his son, the saint is visibly angry. This fuels his conflict with the Straw Hats. It's admirable when parents step up for their children, but that doesn't make up for the subjugation of innocents.
4 Saint Jalmack
The villain of Red resembles Rosward in appearance and worldview. Jalmack is a staunch supporter of classism, having a gaggle of slaves at his beck and call. He even relies on them for trivial matters like carrying him from place to place. Walking is for poor people, after all. That's not exaggerating his mindset.
This "saint" really views lower classes as subhuman creatures. As such, he tries to keep them as far away as possible. That's why he wears his helmet, as he doesn't want to share the same air as commoners. Of course, that's not to say he has a problem mistreating them.
Jalmack abuses his authority without fail. Whoever's on the receiving end, he shows no mercy. The proof comes when he shoots at Sabo: a child waving a pirate flag. He supposedly does this in the name of the law, but anyone with eyes can see that is just an excuse. Hiding behind said law to facilitate such actions is not only a perversion of justice, but it reeks of cowardice. He can't acknowledge his own monstrous nature.
3 Donquixote Doflamingo
This flamboyant leader is the most open with his malice. Doflamingo loves to flaunt his power, both as a captain and warlord. Inflicting pain is his favorite pastime. If he dismembers opponents, for instance, he laughs at their suffering and strives to cause more. Such a direct approach is unusual for him, though.
Most of Doflamingo's sadism revolves around pitting people against each other. He forces a king to slaughter his own subjects and frames the monarch for the massacre. On a more personal level, he may compel two family members to fight to the death. These actions are sometimes about asserting dominance, but they mostly serve as little more than entertainment. Human beings are his playthings. It's just how the pink menace gets his sick kicks.
2 The Five Elders
Among the highest Celestial Dragons in rank are the Five Elders. This slew of senior citizens runs the World Government, but that's not all. They lord over several groups--the prominent ones being the Seven Warlords of the Sea, Cipher Pol, and the Marines. The Elders use that influence to pull the whole realm's strings. They work to maintain order and ensure history's events go according to their plan. On the surface, that doesn't sound as terrible as sadistic Dragons like Doflamingo.
Upon a deeper look, though, the Elders' position and actions demonstrate an unmatched hubris. They think they know what's best for society at large. What's more, is that they're willing to commit genocide to fulfill their grand plans. Granted, they don't view these scenarios as ideal, and they express some remorse as the smoke clears. That said, they still pull the trigger, as they believe that their ends justify the means. Few villains in One Piece play God to this degree. Only a single person beats the Five Elders in that regard.
1 Im
Also known as "Imu," this shadowy figure is the puppet master behind everything. He stands above the Five Elders and the World Government. Despite the mystery of his appearance, they all remain subservient to his will. That makes him worse than them all.
Im is the overarching baddie of the whole One Piece saga. He's indirectly responsible for all of his minions' misdeeds. That means he's behind most of the misfortune that Luffy and the Straw Hats contend with. He's also the reason that so many people face persecution and oppression by those above them. Im has sinned on a magnitude beyond reckoning. That makes him the worst Celestial Dragon by a mile.