Hmm. If I'm going by personal feelings, I'd say Oden or Shinobu, because I don't really like either one:
- Shinobu was helpful once near the beginning of the arc in a role someone else easily could have taken, and has since been mostly useless and reduced to a running gag that's stale and not particularly funny, though I suppose there is a chance she could be redeemed by the end of the battle.
- Oden failed to land with me as a good character and absolutely pales in comparison to the likes of flashback greats like Hiruluk, Saul, Tom, Fisher Tiger, and Corazon. The meme-level introduction we got for him before actually meeting him got things off to a bad start, and he feels like this series's Itachi Uchiha of getting excessive levels of character shilling from absolutely everyone. The things I liked best about his flashback were 1) the parts least focused on him that instead highlighted WB and Roger and their crews, and 2) his death scene, which was pretty well-done. Still, as others have noted, it would be really hard to take Oden out of this arc without completely altering everything, so maybe with him, I just wish his character was different, haha.
But in terms of story function and personal feelings, it's gotta be Orochi. For one thing, I'm quite unimpressed with his motives and I find him to be an even-less-entertaining, even-more-annoying Spandam for this arc.
But also, as others have noted, he's taken away a lot of spotlight and potentially interesting interactions with the heroes that should have gone to Kaido, who's ostensibly the main villain of this arc. We'd been led to believe before Wano that Momo's, Kin's, and company's main grudge was against Kaido, but much/most of what we actually saw during Acts 1, 2, and the flashback in 3 have made it seem like things are way more personal between the samurai and Orochi, while Kaido essentially served as "the main reason the good guys can't just one-shot this fool Orochi and solve all the problems". It was primarily once this raid started that the story really went back to trying to convince the readers that "No, really, guys, Kaido is the one we all hate who needs to go down and Orochi was always inconsequential", but, while this is certainly true power-wise, on a personal level between the characters, it doesn't feel convincing enough after the portrayals of both throughout the arc.
If Orochi's actions and role had been given to Kaido, the latter would function much better as a personally hate-able, "I can't wait to see you fall" big bad of the arc.