Easiest answer, Law cuz he’s the perfect balance of everything you want in a Shonen MC and more.
First off a very good character design, he’s hot so easily marketable to women. He also has great fan ships with Zoro for yaoi and the hottest SH aka Nico Robin for hetero.
He has the brains to lay out long-term plans and get his hands dirty to do it, while still maintaining a moral code that doesn’t make him horribly disliked. He was the catalyst to turning the gears of the NW post-TS.
Balls to do shit that will turn the world upside down and powers to accomplish that. He was willing to distract Doffy and an Admiral while letting SHs escape, fight against a Yonko 2v1 with someone as lackluster as Eustass Midd.
Driven by personal vendetta at times so he's not a Mary Sue either. Let his hatred of Doffy override his overall agenda. Still rational and not above pulling off crazy shit mid-battle.
A "neutral but good" pirate, helps out the weak and poor cuz it's the morally right thing to do, but also recognizes that he's a pirate so he doesn't put up with fame or glory, just want to learn more about lore and history.
Very likable character as a whole, not carried by MC crew status, being Top 2 in popularity during Dressrosa, still remaining Top 5 despite not being a SH (in fact the ONLY non-SH to be in Top 5).
His powers are the best, lots of versatility, potential for creativity, also best of both worlds for fighting style, quick and lethal thanks to teleportation and internal attacks, also having massive AP/DC with super flashy moves. None of that confusing Haki ACoC bullshit.
Simply the goat