LUFFY VS ZORO AS CHARACTERS
LUFFY
Issue with Luffy is that while his character archetype has innate complexity associated with it if we try to analyze his goal. He wants to be Freest Man in the World which is something that can one of the more complex character layers in One Piece. Like what exactly does he mean by Freedom? How much of his freedom means suppression for the rest of the world? Does he want Anarchy because without overthrowing current regime, the world government and ending the Marines he can't be freest man in the world.
But even if he ends current regime, a better regime comes along, said regime would also not let Luffy roam around freely due to his crime. So for him to truly be Freest man in the world he would have to at least over throw any form of authority, and if he does that what does that mean for common people? They would be looted by pirates, no one would be there to protect him? Would Luffy ultimately take all the pirates down as well? Once again taking away hopes, dreams and freedom of the rest of the world. But then what? Does he become sole governing authority of the world who rules with iron thumb and don't let anyone rise above him? But by that logic he is taking away freedom of the rest of the world.
Now Luffy doesn't exactly have any larger than life or even noble motivations for wanting to change the world. Oda hasn't exactly explored on that, which again if he did Luffy could have been one of the more interesting characters in anime, but instead his goal to be freest man in the world just sorta kinda come to him. He just wants it because he wants it, not because of any resolution, not because of any trauma he been through, not because as a kid he has seen too many miseries and sufferings in the world that he wants to end. So what Luffy's character archetype is much more complex than Zoro's, his execution is so flawed he feels sub par character.
Issue is Oda hasn't really explored the most complex aspect of his character and instead focused on him being a bumbling idiot who eats meats and has weird ass interractions with arc princesses and screams "I WANNA BE KING OF THE PIRATES AT THE TOP OF HIS LUNGS" and his character hasn't evolved even slightly in 25 years.
It sucks because Oda doesn't explicitly state Luffy is selfish either despite his selfish motivations, instead he has to be second coming of Jesus christ, hero to people etc.
Oda had potential to make one of the greatest protagonists by not going to usual "retarded shounen protagonist hero"
ZORO
Zoro on the other hand has a simple character archetype but his execution is much better that makes him much more relatable. While he has simple goal of attaining World's Strongest title, his motivation comes from trauma of child hood. He took tragedy of his child hood, lost of his best friend and used it to fuel his motivation. But not only that, he also wants to full fill his dead friends dream, showing nobility.
Both layers are very easy to follow and straight forward. Then he was shown what it means to World's Strongest, how ignorant he is to the rest of the world and he changes his out look on the world, genuinely changes the way he interracts gets a lot more serious and has a chip on his shoulder and focus. Against Mihawk his motivation also evolved into not just wanting to become WSS due to promise to his dead friend and making his name reach the heavens so Kuina can hear him, he also wants to become World's Strongest swordsman for Luffy's sake so he can make him Pirate King.
Then in Thriller Bark his motivation further evolved to now he values Luffy's ambition and protecting his crew above his own. Not only that he believes if he can't protect his captain's ambition he isn't worthy of becoming WSS. And all these changes stick to Zoro, they are consistent through out, once again natural character development.
Not to mention other layers like Zoro being level headed when level headedness and effective leadership is needed like in Alabasta, Whiskey Peak, Thriller Bark, Water 7, Punk Hazard, Wano etc Luffy on the other hand hasn't really shown true leadership, all he has shown is ability to motivate people which is important part of leadership but what happens after he has actually motivated people? When it comes to making tough decisions Zoro always have to bring Luffy to right path. And other layers like kindness behind his tough exterior, arrogance yet awareness of where he is, hatered for atrocities etc.
Its much easier to like Zoro than it is to like Luffy because his character writing is simply much more effective. Luffy just feels like is sorta kinda there with no natural progression. Most interest part of his character is never even explored and all Luffy is is a bumbling idiot eating meat, having weird interactions with princess and screaming "I WANNA BE KING OF PIRATES" at the top of his lungs.