I think Gon is the best main character.
He's a happy, friendly, and innocent. He does what every child does, or want to do. He was doing heroic things, as saving the cub and becomes its friend. He was inspired by Kite to become a hunter, and actually became one. He likes to race people, as we've seen in the Hunter Exam's first phase, and likes to eat a lot.
What makes him more special than that is that he's neutral. He's not the type to say "Let me handle that." "I'm going to do it!" "Don't do that!". We've seen this during the fight between Killua and Johness. He lets his friend fight however that suited them, but he hated killing in the same time. Killua changed to not killing because he hated it, and Gon most likely liked that massively a lot.
He's determined, and somewhat simple. He's smart too. We've seen him using strategies in fights, not just hack and slash, or just punching the hell of your opponent. He also realizes his limit, and acknowledge those in the world with power greater than him. Not just "I will become stronger and defeat you!" "Even if you were a million times stronger, I will take you down!".
He also grew a lot in the recent arcs. He got in a sorrow that left him in a dark world. He was determined to take his opponent down at all costs. The one who was somewhat cautious. He grew darker and darker. He was freaking out at his opponent, and was going to kill someone that didn't have to do with anything for his goal. He grew desperate to the point he gave everything up, and achieved his goal through means worse than death itself! I didn't see, or don't know any main character who reached this point. That's a huge development in terms of his character.
Everyone likes Gon. That's why I think he deserves the #1 spot in terms of main characters.