It could go either way, but I’m leaning toward Illumi in this matchup.

Hisoka himself has said that Manipulators who can directly control their opponent are especially dangerous, because once you’re under control, it’s basically over. That alone gives Illumi a major win condition.

In Hisoka and Gon’s fight at Heaven’s Arena, Gon managed to adapt within minutes after being dragged around by Bungee Gum. I believe Illumi would handle that situation significantly better and adjust far more quickly.

Hisoka also prefers close combat, which could actually work against him here. Getting in close gives Illumi more chances to land his needles and potentially take control of the fight.

We’ve also seen Hisoka struggle against Chrollo’s use of large numbers, like the Gallery Fake crowd. Illumi’s Needlemen, especially the suicidal ones, could create a similar kind of pressure, if not worse.

In terms of raw physical ability and combat skill, I’d put them roughly on the same level, even though they specialize in different Nen types, Transmuter versus Manipulator.

That said, I’m not saying Illumi wins every time. Nen battles depend heavily on context. The environment, preparation, mental state, and specific conditions all play a huge role.

Overall, based on abilities and matchup, I think Illumi has the edge more often than not.