Damn. Sorry this took like hours to type cause I keep getting interrupted.
@The:
Huh, from my experience, Usopp was the most hated character, while Sanji was more in the middle, or at least I didn't see lots of hate for him. Maybe people have warmed up to Usopp more recently then.
Actually you're both right.
As a fan of both characters, let me try to explain it in my point of view.
Usopp is hated by superficial individuals for his appearance and external personality while his growth be denied and/or misconceived.
Sanji on the other hand is the most controversial SH, he is basically hated by many feminists for his misogynistic attitude and people who compare him to his pre-TS self or his initial introduction.
Either way, these people (and Darth) fail to realize that the majority of the characters in the series are flawed - that's what makes them engaging and real, but the SHs grow and influence each other despite of it. After all, we as humans are all flawed and we develop despite of it - either they be for the better or worse, do we not?
I would say Usopp's growth is the most detailed and down to earth. His growth is getting harder to be denied as the series go on, from the very first time he shot a cannon and was surprised by his skill to his internal struggles to step up to fight not only for who he cares about but now for strangers as well ("The Legendary Hero") but people still conveniently forget that he has 2 goals. I was honestly surprised to hear quite a number of people say they haven't forgiven his character for leaving the crew in W7 until this chapter, since not only was that the biggest turning point to his character and his job, there were so many levels on why he left. Usopp felt useless compared to the "monsters" of the crew, not only that but for not being able to do a simple job and keep the money that he was entrusted with. The Merry was his connection to his village and contribution to the crew, it's been made clear that even among the SHs, the ship was most special to him. Lastly, we know that his father left to become a pirate, then shortly after was left alone with his sick, dying mother which resembles the situation with the Merry at the time. Adding all of that, it's easy to see he tried to relieve his weakness and fear of abandonment by leaving the crew himself. As Zoro said, he won't say who was right or who was wrong, but going against the Captain and leaving a crew isn't that simple - that they're not playing a game and what Usopp had to do was simple. Which he did, Usopp apologized in the end by throwing his huge pride away.
As for his sharpshooting skills, it was also in W7 where it had it's biggest turning point or more like the beginning depending on how you see it. His weapon had always been a slingshot but his job as the sniper has always been cloudy, something he just settled for not being able to be the Captain and instead treated himself to be as the second-in-command. I believe it was Sanji's words in Enies Lobby that helped Usopp realize that his fighting style, his position and his skills shine the brightest from the distance. It was no coincidence that Sogeking's skills as a sniper was emphasized and that he rejoined the crew in the same arc that Merry passed. The position of the sniper is something he just can't "settle for", that the position holds importance that only he can fill and that being second-in-command aboard the Sunny is not possible. After which, his own sharpshooting skills were starting to be stressed on, against Perona, Sabaody and finally in Boin Archipelago in which he vowed to become the real Sogeking to aid Luffy.
–-
Now where to start with Sanji... his background. Since he was a child, Sanji was raised by a pirate in Baratie, the sea restaurant. Due to that however, pretty much as expected his communication with only manly men and ex-pirates molded him to grow up the way he is. Sanji's only exposure to female company were customers, ones that he was never raised to harm and serve wholeheartedly. From the very first time he met with the crew, I would say Zoro's attitude and words resonated with him the most. Sanji belittled his own desire to pursue his dream due to his twisted resolve to pay back his savior, to whom he never showed any good feelings or attitude towards, resulting in his own co-workers misinterpretation of their relationship. When he left the restaurant, it was shown that these men actually cared about him, that they showed it in a way that they only know how to, similarly to himself.
His rivalry with Zoro as well as his massively shifting attitude towards the male and female crew members was his own way to show he cares for both alike - the only way he knows how and the same way he was raised. We've seen plenty of evidence of this: when he dived in first in Arlong Park, his retrieval of Usopp's goggles in Alabasta, his sacrifice in Skypeia despite saying that he wouldn't mind Usopp's death for Nami's sake, his clear emotional rage towards anyone who badmouthed Robin in W7 and Enies Lobby, letting Absalom attack him in return for Nami's safety. The most clear evidence however was his intentional sacrifice for his rival, he knew how much Zoro's ambition meant to him and that he believed in him to succeed in achieving it.
Now, Sanji's attitude towards women is his flaw but you can't exactly deny that it's consistent to his character and his own resolves either. I understand a lot of people lost respect to him when he couldn't attack Kalifa, but Nami one of the very women he, himself thinks he has to protect, stepping up for him is a good change for him (which was touched on with his conversation with Franky on Dressrosa). Not only that but his words to Usopp (and I believe to himself) shortly after was his own way to redeem his actions for Robin's sake. Usopp's weakness to fight Jabura but his skills as a sniper and his own inability to fight women and the honor to rescue Robin. I believe Sanji understood all of it and left it up to Usopp to save her, which highlights Luffy's words in Arlong Park, as well as the SHs mindset as a crew - to be able to do what others can't and to which, he proved when he used the lever at the end of Enies Lobby to close the Gates of Justice.
After the time-skip Sanji has hit a clear cut difficulty to perform per his resolve, becoming incapacitated then almost dying due to a nose bleed, saving a woman only to be beaten by Vergo, trying to save his crew, again only to be beaten by Doflamingo. It's not far-fetched to say that Oda is doing this to test his resolves even further and I, as a fan am glad.