Gon Freecss' journey across Hunter x Hunter's world wouldn't really mean anything without some adversity. Finding his dad, seemingly the world's greatest Hunter has to feel like a true achievement, and that means putting oneself under great challenges to find him.
More often than not, this has meant putting Gon against all manner of dangerous villains, all for the sake of getting strong enough and surviving long enough to meet his dad. If this seems counterintuitive, it was. Gon has barely survived his journeys across Hunter x Hunter, and none of the opponents that he's faced have ever shown a sign of showing mercy.
10 Baro
Baro was one of the earliest Chimera Ants in the series and one of the first that Gon ever got to fight. He was an armadillo-type of ant that rolled and thrashed around like Choji from Naruto. He certainly wasn't a conventional opponent for Gon; and as a Chimera Ant, he was a soldier of slaughter for the king's whims.
Unfortunately for him, he didn't get to do much slaughtering. As dangerous as Baro could've been, he has the honor of being the first person that Gon ever got to kill. It was a mighty step for Gon and a huge plunge downward for Baro.
9 Rammot
Rammot was a much more hostile, much more versatile ant than Baro. Not only would he act independently from his troop, but he also explicitly went after Gon and Killua after being handed a decisive defeat by them. At his disposal are the strengths of a rabbit and shrike, and the Nen ability to turn his feathers into deadly blades.
As an aggressive figure who went out of his way to attack the boys, Rammot was certainly a huge leap from Baro when challenging the Chimera Ants. Fortunately for Gon, Killua would handle him singlehandedly to deliver an unceremonious defeat.
8 Gido
Gido was an experienced fighter at Heavens Arena who was desperate for wins to climb up the rankings. Noticing that Gon and Killua were fresh meat in the world of Nen, he took it upon himself to break them into the true ranks of Heavens Arena while simultaneously scoring himself some easy points.
During his first encounter with Gon, the little kid did his best to evade Gido's attacks but the much more experienced fighter always had a trick up his sleeve and a spinning top ready to attack. The first confrontation would end with Gon near mortally injured from Gido's relentless attack. However, Gon would exact revenge during the two's second encounter, being more powerful and experienced than ever.
7 Binolt
Binolt was likely one of the earliest characters that would have truly killed Gon if he ever got the chance. As a historied serial killer and a long-time player of Greed Island, Binolt was a quick and ravenous fighter just ready to cut his opponents' throats. Despite his clear instability and experience over Gon and Killua, Biscuit Krueger though Binolt was the perfect sparring partner for the boys' training.
Gon would even get the chance to fight Binolt one-on-one and delivered the killer a massive and painful defeat. Fortunately for Binolt, Gon took mercy upon him for helping them train, and the boy decided just to let him go.
6 Neferpitou's Kite Puppet
Kite himself was the furthest thing from a rival or a villain to Gon. He was on old disciple of Ging who found it an honor to train the famous Hunter's son. However, as great of a teacher as Kite was, he'd sadly die early on in the story when the Chimera Ant Arc kicked into its next gear. That's where the Kite Puppet comes in.
After experimenting on her strongest opponent yet, Neferpitou resurrected Kite's body as a sparring puppet that would lash out violently at the closest thing to it. Gon would quickly realize that this thing wasn't the Kite that he once knew, though he'd voluntarily take a quick beating to learn that.
5 Genthru
Genthru was a player in Greed Island who had gotten sick and tired of forging through the game. Using the gifts that Nen gave him, he and his cohorts decided to threaten other players for their cards, quickly killing off anyone that would try to betray them. His thievery and willingness to kill other players without remorse made Genthru the Greed Island Arc's top villain and a clear target for Gon.
Up until that point, Genthru was the most experienced and hostile Nen user that would take Gon up for a serious fight. This wasn't just for points either. This was a life or death battle to collect some cards, and Genthru really did Gon work for it.
4 Pakunoda
Pakunoda was one of the key members of the Phantom Troupe during the Yorknew City Arc. Much like the rest of the Troupe, Pakunoda had superhuman strength, expert stealth skills, and a profound mastery of Nen. She also benefited from being a quick shot with a revolver which made using her Nen ability even easier.
When Gon and Killua tried to spy on the Troupe for money, she was one of the agents that helped catch them. While Killua was trying to escape from Phinks, Gon was getting easily defeated by Pakunoda.
3 Nobunaga Hazama
Nobunaga Hazama is another member of the Phantom Troupe who took a special interest in Gon after Pakunoda and Phinks captured him and Killua. Perhaps taking some inspiration from his namesake, Nobunaga was a master swordsman who acted as one of the Troupe's vanguards alongside Uvogin.
It wouldn't be until later that the community would learn that Nobunaga was actually the #1 ranked member of the spiders, making him perhaps the most dangerous member behind Chrollo himself. He was in charge of keeping Gon and Killua captive while the rest of the ants were away. He watched the boys intently and was able to block any move that they had. With few options, Gon and Killua had to get creative with their escape.
2 Neferpitou
Neferpitou, as a member of Meruem's Royal Guard, was one of the most powerful and dangerous of the Chimera Ants. Neferpitou's sheer physical strength alone was more than a match for some of the Hunter Association's best. Between their sharp claws and pouncing strength, Pitou is a fearsome predator.
On top of that, Neferpitou is also an exceptional Nen user. Their En alone far exceeds the sensory capacity of other masters. As a specialist, they're able to heal injuries and operate with ease, control people like puppets, and even enhance their already incredible strength. Gon needed to unlock Adult Gon to be able to beat them.
1 Hisoka Morow
Even though Gon has fought against hyper-evolved ants that literally eat human flesh, no Chimera Ant was ever quite as ruthless or even creepy as Mr. Murder Clown himself, Hisoka Morow. Hisoka is a killer that revels in his opponents' blood and fighting strong enemies.
Even in the middle of a fight where he's sustaining noticeable injuries, he never loses his calm demeanor. He can turn the situation quickly into another one of his deadly games. It doesn't help that he's quick learner and has some of the most versatile Nen abilities in the series. Many shonen protagonists love to be competitive with their rivals, but Gon always seems like he wants to put a restraining order on his.