One Piece is one of the longest-running anime series in history. It spans over a thousand chapters, and yet it doesn't feel dragged out. The characters in One Piece are nicely fleshed out, which makes them fascinating. This applies to both heroes and villains.
In fact, many villains in the series adhere to a strong belief and never change it. These villains do not commit crimes out of the blue. They have a reason that pushes them to act in a certain way. In fact, it could perhaps be argued that some One Piece villains actually had a point.
1 Arlong
Arlong might not have been the strongest villain in One Piece, but he represented something far more important. As he grew up on the bad side of Fish-Man Island, Arlong learned to hate human beings. He would hear stories about how humans mistreated Fish-Men.
Growing up, Along witnessed the horrible deeds of humans firsthand. Due to his hatred for them, he never trusted another human. He tried to protect members of his own race from harm. In his mind, Arlong did nothing wrong, but he certainly went about it in the worst way possible.
2 Kaido
Kaido was the main villain of the Wano Country arc. He was the captain of the Beasts Pirates and one of the Yonko. Kaido managed to garner the reputation of being the strongest creature in the world through sheer power. Throughout the arc, Kaido demonstrated his strength and how superior he was to the other characters in Wano.
Kaido was obsessed with war, and he wanted to start the biggest war that the world had ever seen. Initially, it seemed like Kaido was just a brute, but there was a major reason behind his behavior. Kaido strongly believed that people could only be equal in war, which is why he always tried to start fights.
3 Akainu
Akainu is possibly the most hated character in One Piece, mainly because of his involvement in the Summit War, during which he killed Ace. He is currently the Fleet Admiral of the Marines, which means that he has command over every single Marine. Akainu is an ardent believer in Absolute Justice and thinks that nothing should obstruct the path of righteousness.
The biggest problem with him is that he is unwilling to compromise justice. He doesn't even spare his underlings or any innocent civilians, for that matter, if they try to go against justice. While his methods are definitely tyrannical, Akainu knows that in a world of pirates, it is important to be absolute.
4 Blackbeard
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Blackbeard is considered by many to be Luffy's final opponent. Blackbeard and Luffy are two sides of the same coin, which is why it is always interesting to see the former's perspective on things. In comparison to Luffy, Blackbeard is a calculating man who only takes action if it suits his personal interests.
However, there have been certain occasions throughout the story where Blackbeard acted rather recklessly, and it was due to his strong belief in fate. When things go wrong, Blackbeard shrugs them off as a work of fate. His way of thinking even inspired a cold-blooded killer like Shiryu to join the Blackbeard Pirates. Blackbeard also believes that a person is free to dream, and, in his eyes, they shouldn't stop at anything.
5 Charlotte Katakuri
Charlotte Katakuri is one of the Sweet Commanders of the Big Mom Pirates. He has one of the best introductions in the series, in which he kills a man with a jelly bean. This moment, along with several others, boosted Katakuri's popularity within the fanbase.
The Sweet Commander proved to be the biggest obstacle in Luffy's path on Whole Cake Island. For the majority of their fight, Katakuri easily pummeled Luffy, and there wasn't much the Straw Hat could do about it. Katakuri's reason for fighting was to protect his siblings from harm. He believed that his duty to his family came before everything else.
6 Gecko Moria
Gecko Moria is the captain of the Thriller Bark Pirates. He was the second Shichibukai that the Straw Hats fought after Crocodile. Gecko Moria possesses the Kage Kage no Mi, which is a Paramecia-type Devil Fruit that allows him to control shadows. In his heyday, Moria was able to match Kaido in combat, which is definitely a big feat.
During their confrontation in Wano, Kaido obliterated Moria's entire crew, forcing the latter to retreat. Moria was saddened by the loss of his crewmates, so he decided to create underlings who would never die. In order to do so, Moria used the power of the Kage Kage no Mi and stole the shadows of other people. Moria took the wrong approach, but he only did so because he did not want to lose his crewmates ever again.
7 Smoker
Smoker is a Marine Vice-Admiral who was heavily involved in the story at one point. He made his debut in the Loguetown arc, and he nearly captured Luffy before the pirate could set foot in the Grand Line. Luckily for the pirate, Dragon managed to stop Smoker in the nick of time.
After letting Luffy slip through his fingers, Smoker became obsessed with capturing him. His manhunt for Luffy is driven by his desire to prevent the pirate from toppling the World Government. Smoker knows there are similarities between Luffy and Roger, so deep down, the Marine is aware that if Luffy isn't stopped, the entire world will enter a state of chaos.