Bellamy is a good character and his resolution in Dressrosa came as a breath of fresh air in comparison to all of those people whose development consists on crying and giving up to Luffy's help. On the contrary, Bellamy had the courage to deal with his personal flaws and ethics and faced the consequences of his choices as the path he needed to follow to achieve peace with himself and ultimately redemption.
King has the most repetitive, cliché and uninteresting design among the Disasters. And his personality isn't that much attractive to me.
Brook and Franky are Straw Hats that Oda didn't really know how to handle in terms of their gags and I think he still doesn't. Back when they joined it felt like Oda was experiencing some meta phase as a writer and would give them gags consisting on breaking the fourth wall (Franky's old running gag consisted on stating how he was doing this week, while Brook would joke with things like his To be continued... line). Franky used to sing and play a guitar, too, which is missing nowadays, while Brook has this pervert running gag that I've always found to be repetitive having Sanji in the crew. In fact, Brook works the best when jokes are made from the fact that he's actually kind of crazy after his decades of isolation (when he was sleeping with Big Mom all relaxed, when he bursted into laughter at Duval looking like Sanji, etc.). If Franky retained his meta-gags (and recovered more of his gang personality) and Brook lost his pervert side I think they both would work better (I mean, Brook showed his crazy side during many instances in Whole Cake and they were some of his best moments in the story overall).