Blacklist Hunters are some of the most powerful Hunters in the world of Hunter x Hunter. They are quite similar to your normal bounty hunters and specifically exist to deal with criminals with a reward on their heads.
As such, their skills with abilities such as nen need to be exceptional. Blacklist Hunters are, thus, quite dangerous in combat. While not many of their ranks have been shown in the series so far, the few that the fans know are quite dangerous.
6 Seaquant
Sequent works for the Zoldyck family and was introduced in the series as their groundskeeper. He is known to be a skilled user of nen, however, his proficiency with this power has not been seen just yet.
He successfully passed the hunter exams and he then went on to become a Blacklist Hunter. The fact that he was able to go past the Testing Gate and face Canary shows that he does possess quite decent fighting capabilities. However, out of the many Blacklist Hunters seen in the story so far, he is certainly one of the weakest. Nonetheless, his true abilities remain unknown to this day.
5 Lippo
Lippo is a Blacklist Hunter introduced quite early into the Hunter x Hunter series. He was first seen acting as the examiner for the 4th and the fifth rounds of the 287th Hunter Exam. Lippo never showed his strength to the fans, however, the fact that he was allowed to be an examiner proves that he is quite skilled in the usage of nen, at the very least.
Furthermore, Lippo was surrounded by skilled users of nen, such as Hisoka and illumi, among many others. For him to be appointed the examiner for important phases of the exam goes to show that he was skilled himself. His work as a Blacklist Hunter remains unknown so far, however, he has most likely gotten his hands on a few powerful criminals.
4 Bushidora
Bushidura was another Blacklist Hunter who was introduced quite late into the series after the Chimera Ant arc. As revealed in the following arc, he was the Hunter Association's disciplinary committee member and he strongly advocated for the revision of the "10 Rules for Hunters."
As a Blacklist Hunter, he was most certainly very skilled in combat and the fact that he held a high position in the Hunter Association means that his skill wasn't all that bad. Unfortunately, he was not seen in combat in the story. Bushidora ended up dying before the conclusion of the 13th Chairman Election. His fight against Hisoka was completely off-screened, and all fans saw was his hat at the latter's feet.
3 Binolt
Binolt made his debut during the Greed Island arc of Hunter x Hunter. According to Bisky, his strength was comparable to a D-Rank-level monster in Greed Island. Before Gon and Killua underwent their training under Bisky, he was known to be stronger than them. Binolt's ability sees him rely on a pair of swords using which he cuts his opponents' hair and upon consuming it, learns everything there is to know about them.
Although he is a criminal, he was also a Blacklist Hunter once and, as such, possesses decent control over Nen. His skills were far above the level of Gon and Killua before their 10-day training. Furthermore, he could also use Zetsu and Hatsu, as seen in the Greed Island arc. Unfortunately, his skill as a Blacklist Hunter was not shown to the fans, however, given his level of strength, he certainly would have been dangerous, especially for the Rookie Hunters who stepped out into the world to make a name for themselves.
2 Saiyu
Saiyu is one of the most skilled Hunters introduced in the story so far. He is a Blacklist Hunter and also a member of the Zodiacs of the Hunter Association under the code name Saru. Shortly before the expedition to the Dark Continent, he is chosen as a part of the Defense Squad, however, he also has strong ties to Pariston Hill.
His abilities as a Blacklist Hunter are certainly considered to be dangerous. In the past, he has openly challenged Ging, who is one of the five strongest Hunters in the world at the moment, which goes to show that his nen control is immense. By the looks of it, he is exceptionally skilled in Conjuration based nen abilities. This was especially evident when he stopped and attack towards Pariston during the Chairman Election arc. Saiyu's abilities have not been fully explored in the series so far, however, in the future, fans will certainly get to see what he's truly capable of.
1 Kurapika
Kurapika is one of the main characters of Hunter x Hunter. He passed the Hunter Exams along with Gon and Leorio and ended up learning nen quite quickly. He is known to be an exceptionally skilled user of Conjuration nen techniques. Kurapika ended up becoming a Blacklist Hunter and tracking down those who were involved in the murder of the Kurta Clan.
At the same time, he has also made it his mission to retrieve all the eyes of his clan members, which are highly coveted. Kurapika is an incredibly dangerous Hunter and, quite possibly, one of the strongest in the entire world. In the past, he has defeated strong fighters, such as Uvogin of the Phantom Troupe. Even now, his full capabilities remain unknown and he will certainly surprise fans more as the Succession Contest arc continues.