This is an episode that really develops Gon's character. He shows the essence of what being a Hunter is all about. Gon's resolve to tackle challenges that are seemingly too difficult to him becomes a common theme in the series and this is one of the brightest moments. I see this episode as an encapsulation of the entire series itself. Gon, on his journey to hunt an ever-elusive and powerful target, his father. In this episode, Gon is on a mission to hunt an ever-elusive and powerful target, Hisoka.
If someone said to pick a quintessential episode, maybe not the best, but quintessential episode of this series, it would be hard to not argue for this episode. It's that good.
And also, if the manga ever completes, let's be honest with this question: what is the one-on-one fight for all the marbles that should happen for closure and completion of character arcs? Watch this episode to get the hints.