Content Warning: This article contains discussions of murder and violence
As the starting TV show of the Arrowverse, Arrow featured a lot of villains (some of which later appeared in subsequent shows). Unlike other superheroes in the franchise, Oliver Queen had no special powers, which meant a selection of his enemies decided to target him because they saw this as a weakness. A large majority of them also didn't possess any extraordinary powers and were simply intelligent and/or well-trained in combat (such as Prometheus, Dark Archer, or even his sister Emiko).
While many fans knew that good would ultimately prevail over evil, there were a few occasions where the villain actually came close to besting Oliver. In fact, due to their conniving and careful planning, some villains managed to weaken Team Arrow by either killing a member or placing doubt among them. Whatever the case, a lot of the Arrow's villains presented a real threat thanks to their intelligence.
Emiko Queen
Emiko is far from stupid. However, her biggest mistake was letting her emotions, rage, and thirst for revenge blind her. Emiko was so angry at her father for abandoning her she was willing to betray her brother, who tried to make things right.
Plus, she didn't recognize that her so-called allies were lying to her about the death of her mom. Luckily, Emiko got a second chance at life thanks to Oliver's sacrifice so she has a strong potential to turn over a new leaf.
Ra's Al Ghul
For such a respectable figure, centuries-old warrior, Ra's al Ghul made several great mistakes. One of them was letting Malcolm Merlyn escape his grasp. Ra's should have realized Merlyn would never stop plotting against him.
Another mistake was to overlook his daughter Nyssa's growing discontent with her father's behavior. And finally, Ra's made the mistake of trusting Oliver, who infiltrated the League of Assassins. All of this and more led to this villain's downfall.
Deadshot
Despite his line of work and dark past, Floyd Lawton (aka Deadshot) became one of the more popular villains. He was responsible for supposedly killing John Diggle's brother Andy.
Even Lawton's skill didn't stop him from getting captured and subsequently serving in the Suicide Squad. Lawton sometimes worked alongside the heroes, especially John and Lyla, and he ultimately sacrificed his life to save the team when a mission went wrong.
Black Siren
From a villain to a hero, that's not a road many Arrow villains followed. But the Black Siren is the exception to this rule. She made several questionable decisions, such as when she decided to revert to her old ways when it looked like nobody believed that she didn't kill Ricardo Diaz.
However, it seems that Laurel has definitely turned over a new leaf by the end of the show and she will be an important member of the Canaries in the future, which requires to be cunning and brave if nothing else.
Ricardo Diaz
Ricardo Diaz is one of the villains who lasted on the show for a long time. He became a crime lord, with a large number of people working for him. That gave Diaz the means he needed to pursue his goals.
He was intelligent and ambitious enough to take over Star City's law enforcement, for example. And unlike other villains, Oliver never succeeded in killing Diaz.
Damien Darhk
Damien Darhk brought magic into the show, something the ever-so-practical Oliver had trouble dealing with. In fact, it took him multiple tries and encounters until he defeated Darhk.
In the meantime, Darhk was able to lead his own organization, threatened Quentin Lance into helping him, and succeeded in killing Laurel Lance (aka the Black Canary). And even though Oliver killed him in the end, Damien was later resurrected and appeared again in different Arrowverse shows, such as Legends of Tomorrow.
Prometheus
Also known under his civilian name of Adrian Chase, Prometheus was able to fool even the ever-so-distrustful Oliver. They worked side by side and Oliver didn't know he was talking to his enemy.
The moment of his greatest triumph came when he succeeded in killing Samantha Clayton, William's mother. Even though it led to his death as well, it left a lasting mark on Oliver's life. He also managed to persuade Evelyn Sharpe to betray the team Arrow and work for him instead. As a result, he discovered the identities of the whole team. What's worse, Prometheus even managed to manipulate Oliver into killing Felicity's boyfriend Billy Malone. He almost tore the team
Deathstroke
Slade Wilson is one of the most popular Arrow villains - and for a good reason. He originally started out as a good (albeit somewhat grumpy) guy and a mentor to Oliver on Lian Yu.
When the Mirakuru serum affected his mind, Wilson became violent and determined to get revenge on Oliver for Shado's death. He not only managed to survive but also manipulated the Queen family. In the end, he killed Moira, who offered her life in exchange for her children's lives.
Dark Archer
Malcolm Merlyn is the first major villain on Arrow. He's highly intelligent, manipulative, and adaptable. Malcolm always seems to have a plan B and if that doesn't work, then plan C. He partially succeeds in his plan to destroy the Glades, even though Oliver and his team stand in his way.
He manages to get Oliver involved in the League of Assassins', which leads to Ra's al Ghul's death. Who knows what else could have Malcolm achieved if he hadn't sacrificed his life for his daughter Thea.