Hisoka and Illumi are among the strongest characters in Hunter X Hunter. They are both good friends and a lot of us speculate as to who is the stronger among the two.
Now, we have seen Hisoka fight many times in the anime and Illumi has yet to show his true potential. They are both considered to be one the same level as fighters even thought we haven’t really seen Illumi fight yet.
Hisoka is a transmitter who is an extremely skilled nen user. He uses bungee gum and cards against his opponent. He is extremely tactful and clever in his fighting style however, one could say that he does get careless sometimes like in his fight against Chrollo and Kastro. Kastro was able to cut Hisoka’s arms but lost the fight. Hisoka could have easily avoided getting his arms cut had he wanted. Hisoka lost his battle against Chrollo. Chrollo was too overpowered and was using all the Phantom Troupe member’s nen techniques to beat him. Hisoka is quite talented and was able to catch Togari’s blades without any practice. Togari took over half a year to do so. Hisoka has incredible hand to hand combat skills, speed and is quite smart as a person too. After his fight with Chrollo, Hisoka gets much stronger too. He is able to kill Shalnark and Kortopi within no time. Post death nen makes the user stronger than before. Thus, Hisoka is even more powerful than before.
Illumi is the eldest son of Silva Zoldyck. He is a manipulator and uses needles to control people just like puppets. The controlled people follow his orders until they die. Apart from that, Illumi is a great assassin since he is a part of the Zoldyck family. He is resistant to poison and has high tolerance to pain. He seems unaffected after his arm is broken by Gon. Illumi has great speed and hand to hand combat skills too. We see some of his skills in the movie ‘Phantom Rouge’ against Gon. He is a puppet, thus his power shown in the movie is much less than his actual strength in the anime. However, that does give us a good idea of how strong he is. Considering that he is trained by Silva or Zeno or both, I believe he is very strong.
Hisoka rates Illumi’s power at 95.
Now, Hisoka did rate Illumi’s power at 95(during the election Arc when Illumi appears as Gitterackur) which a lot of people feel is 5 points below Hisoka’s supposed power level of 100. Considering that Illumi might be weaker than Hisoka. A lot of people believe Hisoka is overestimating himself which might be possible. Perhaps, Illumi was hiding his true strength. In the Gitterackur form, it could be that Illumi might be slightly less strong than his true form.
However, Illumi does not really feel threatened by Hisoka considering that he knows Hisoka likes to challenge strong opponents like him. Hisoka does warn Illumi to not kill Gon or else he would have to face him in a battle. Illumi is wary of strong people and wanted to finish off Gon as soon as possible. Illumi agrees to not killing Gon. Now, one could say that Illumi is sensible and does not pick up fights recklessly like Hisoka does. Illumi fights only when it is necessary to do so. He threatened to kill Hisoka when Hisoka asked him about killing Killua. So, both the fighters are pretty much at the same level.
Who would win in a fight?
It depends on a lot of factors. For example, if it is a crowded area, then Illumi would have huge advantage using people as weapons against Hisoka. We saw how Hisoka lost to Chrollo in a similar situation. However, if it is place with no people and just the two of them, then Hisoka would have an advantage. It is hard to tell how Illumi would win the fight in such a scenario. If we consider Hisoka after his fight with Chrollo, then Hisoka would very likely win since he is much stronger now. If we consider him before his fight with Chrollo, then it is quite hard to predict.
Considering Hisoka has more experience in one on one combat than Illumi (although we don’t know that for sure too), I think Hisoka would most likely win the fight against Illumi.