Unlike many of his fellow DC superheroes, Oliver Queen aka the Green Arrow doesn't have the added advantage of special powers that would help him fight his enemies. Despite that, it's not a good idea to face him since Oliver is not only well-trained and smart but also has a lot of friends who can give him a helping hand when the situation is tough.
That didn't stop some of his villains from coming back over and over again to fight him. And even though Green Arrow's comic book enemies might not be as popular as some of Batman villains, for example, they still often present a real challenge thanks to their large myriad of powers.
Constantine Drakon
Despite his impressive abilities, Constantine Drakon hasn't had as much space in the comics as he would have deserved. That's the main reason he ranks relatively low. It doesn't change the fact that he was able to defeat Green Arrow in combat when they first met.
Drakon is an assassin for hire and when he came to Star City to do his job, he and Oliver crossed ways. As an accomplished martial artist, Drakon was able to win this one fight. He even killed one of Connor Hawke's friends and used Oliver's arrows against him.
Onomatopoeia
The villain with the least pronounceable name is relatively new - and unknown as well, at least in comparison to others. Onomatopoeia proved the old rule that the most dangerous villains are often the ones who enjoy killing. He hunted superheroes for fun and sport, including Green Arrow and Speedy, since he focused on those who had no special powers.
Even though his habit to speak only in sound effects - made him look funny, he was a brutal killer with no mercy for his victims. And when he and Green Arrow fight, Oliver wasn't able to defeat him without help which speaks volumes about Onomatopoeia's skills.
Komodo
One of the ways how to bring a superhero down is to hit him where it hurts. That usually includes going after his relatives or loved ones. Komodo had the right idea when he kidnapped Oliver's sister, lied about killing his father, and also killed Oliver's employees and friends. Even though he later met his demise, he still managed to leave a mark on Green Arrow's life.
He also had an interesting history - as Komodo used to be a former protégé of Oliver's father Robert. He was a capable archer too, very much like Green Arrow, and had formidable fighting abilities.
Brick
Unlike other villains, Brick did appear in the Arrowverse but he was very different from his comic book counterpart. In the comics, Brick is a metahuman whose body is made of stone-like material. He was a much more prominent figure since he led the Star City underworld for a time and united the local criminal clans and families. As such, Brick and Green Arrow crossed paths multiple times, and they were never the best of friends, to put it mildly. For example, Brick one time hired another villain to blow up Oliver's house.
On the other hand, Brick also had a plus side to him since he tried to actually keep peace in the city after a large part of it had been destroyed by other villains, Malcolm Merlyn and Doctor Light. So even though fighting Brick presented a challenge for Green Arrow, there was more to him than just being an evil person.
Cupid
Cupid didn't come across as very compelling on the show but she works better in the comics. Oliver saved her from her abusive partner by killing him but Carrie Cutter, which is Cupid's civil name, was already beyond saving at this point.
She took on the name Cupid, started stalking Green Arrow, and since she was jealous of Black Canary, Green Arrow's longtime love, she tried to kill her. Cupid did know how to fight and even managed to defeat both Malcolm Merlyn and Brick which makes her a force to be reckoned with.
Doctor Light
Doctor Light was one of the few villains who managed to destroy Oliver's beloved Star City. Doctor Light and Green Arrow fought multiple times, and thanks to his large array of powers, Oliver didn't have it easy to deal with him.
As his villain name suggests, Doctor Light can control and manipulate light, fly, shoot energy blasts and even create energy constructs.
Chien Na Wei
Another Green Arrow villain who made it into the Arrowverse was one of the most instrumental people in the comics. Since if it wasn't for her, Oliver Queen never would have ended up on Lian Yu.
At the same time, Chien Na Wei (also known as China White) successfully led her own criminal team and used them for plotting and realizing assassinations, drug-dealing, and other crimes. Combined with her fighting abilities, she's one of the most dangerous opponents Green Arrow has ever faced.
Count Vertigo
Multiple people had the name Count Vertigo in the comics but the current one is called Werner Zytle. He and Green Arrow fought when Green Arrow went to save Shado who had been kidnapped. For a while, it looked like the fight wouldn't end well for Oliver.
However, it's not his fighting abilities that make Count Vertigo such a threat. It's his ability to create and spread the drug that destroyed countless lives which is something even Oliver himself wasn't able to stop from happening.
The 9th Circle
The 9th Circle is one of the newer Green Arrow villains. It's a well-hidden organization, a bank that funds criminal activities. It might not sound like much but they made Oliver's life much more difficult. At one point in the comics, they bankrupted Seattle, and then they took over the city and renamed it Star City.
They destroyed Oliver's company and his reputation in the process. So even though the 9th Circle isn't the type to run around beating people, they still are one of Green Arrow's most dangerous enemies since they operate from the shadows and it's difficult to bring them down.
Malcolm Merlyn
Considering Merlyn's years-long archery experience, it's no wonder that when he and Oliver fought multiple times, Oliver lost more than once. And to this day, Malcolm Merlyn remains Green Arrow's greatest enemy.
Since he has no problem with killing people, unlike Oliver usually does, Merlyn is brutal and merciless in a way Green Arrow was never able to achieve. Merlyn fought multiple superheroes, not just Green Arrow, but they stood against each other often enough - as Merlyn, for example, managed to frame Oliver for a series of murders.