It's hard for me to think of a character whose potential has been ruined and has been treated as awfully as Big Mom, especially during Wano arc, with Oda bringing her in an arc in which she shouldn't have been only to keeping clowning, nerfing her and treating her atrociously with that retarded waterfall issue, amnesia plot and everything that happened to her at Onigashima in an incredibly poor excuse to remove her as an antagonist. The story being now in Elbaf, with which she has so many and strong ties, only reinforces my bitterness about Oda's writing of her.
Tashigi has been treated and wasted horribly as well during Punk Hazard arc, with her keeping losing, being humiliated and needing to be rescued by stronger men with her needing Zoro's help to finish Monet off, her being relagated to a babysitter for Punk Hazard's children after, and her doing nothing as part of Garp's team on Beehive. She could have been far far more, and it really saddens me to see what little Oda has done with her.
Speaking of BM, it's hard to not also think about Charlotte Smoothie who did almost nothing during Whole Cake Island arc aside from some splash attacks and being absent from all onscreen action and not displaying her real power. And that terrible trend continued in Wano where Oda once again, and even more, didn't do anything with her, with him using the aforementioned retarded waterfall issue as an excuse to keep her in limbo. It's jarring how a video game, Pirate Warriors 4, did a lightyears better job at using her and showcasing her power than Oda ever has and ever will in the manga.
And since we are in Elbaf I couldn't not mention Usopp who, aside from during Dressrosa arc where he has some of his brightest moments, didn't do anything since the timeskip and even regressed as a character, with him having yet to do anything in Elbaf arc, which was supposed to be his arc finally.
Oh and special mention for Apoo and Hawkins during Wano arc, with them being relegated to mook commanders in Kaido's crew, whose powers with amazing potential were wasted, and them being anticlimatically defeated.
And how I could forget Toki. She came directly from the Void Century and thus must have some priceless knowledge of the world at this time, even the culture shock of her visiting an entirely different world would be interesting, and she could have been written as an interesting and strong woman. Instead she's reduced to Oden's wife and a time travel device.