Law's arc (Dressrosa) -> Law from a cool and dark guy becomes Doflamingo's punching bag in every fight they had, gets annihilated and literally tortured, he has to beg Luffy to step in to save him and beat Doflamingo despite the latter being his personal nemesis
Sanji's arc (WCI) -> Sanji turned into a bitch and failing to to anything remotely relevant strength-wise, with the meme of "baking a cake" being his most relevant achievement in the arc
Zoro's arc (Wano) -> Zoro admittedly showed great feats and having great portrayal with the Rooftop battle, also achievedCoC and Adv CoC which was crazy, and beat a Yonko's right-hand man... but still not accomplished the maximum goal (ZKK)
Franky's arc (Egghead) -> Franky basically doing nothing except threatening a Seraphim with his laser attack (which he likely would have been able to land only due to the massive advantage in numbers) ... and then getting taken out as a fodder in the most humiliating way, bye bye cybernetic power ups and what not
Better hope your favorite character doesn't get "his" or "her" arc...