There's too many to list... Big Mom would easily be my number 1, but other than her I'd pick any Straw Hat (aside from Luffy), and probably say Usopp is the most wasted/forgotten.
I thought he was supposed to be the character that showed that human ingenuity can get you past any obstacle, regardless of your strength. All of his past fights were won through wit and creativity, but now he just shoots a plant once every while, which does nothing, and then he screams. He used to be able to make the different versions of the clima-tact, incorporate Skypiea dials, and really just use any random object for some purpose or another (even an elastic band in a fight, lol). The idea that he'd get Conqueror's at some point - while I understand why people think it - is kind of laughable with how he's acted in the past 500+ chapters.
It's especially bad considering Franky is the perfect character to pair up with him, theoretically able to make anything Usopp can dream up. If Oda just grouped up the Straw Hats, there's no limit to the different creative fights you can write. Either he's kind of bored of the characters (which, admittedly, is understandable to a degree), or he just doesn't want to put in the effort. It'd be one way to create the "goofy", funny, imaginative fights he seems to want though - he didn't have to give Luffy hints of toonforce for that.