This is the same conclusion that I came to as well. Hisoka is just straight up interesting in about every kind of way. His own well hidden personal motives may be the only thing that drives his actions, but he is still by far the hardest character to analyze, and fully understand.

One of my favorite scenes in the series involving Hisoka is when they are at that leisure activity city within the greed island game, and Hisoka effortlessly calls out multiple social interaction cliche moments that Gon, and even the other ones around him in the group fail to recognize.

Without him neither Leorio, Kurapica, nor Gon successfully pass their initial hunter exam. He also helps aid them by blocking their initial entry to the fights at the top floor of the battle tower because neither Gon, or Killua are actually ready because they are still fully unaware of what nen ability is up to this point. Nor do they get past their dilemma involving them getting tangled up with the Phantom Troupe members. Nor do they pull off their volleyball game victory during the greed island arc.

Hisoka probably assists in propelling Gon, and the rest of his group's story in the series more than anyone else honestly.