Hmm...well, you kind of want it to be someone tough, like Zoro and Sanji, because that's more intimidating. But, they're both down in the Underworld Layer with Luffy. So, they feel less likely. It seems more likely that Luffy, Zoro, and Sanji will have to team up to fight someone else, once they get to where the main action is.
Alternately, you kind of want it to be someone cute, or unassuming. Like Chopper or Nami. Just for the stark contrast. And to show how powerful these "devil" forms make you, if they can make a "weaker" character on par with some of the stronger guys. That better showcases how this is a "upgrade".
So, you know who kind of fits perfectly right in between both categories?
Brook.
He's probably the strongest of the weaklings. He has skill, and some really unique powers, but he just doesn't quite make the cut for the top half of strength among the Strawhats. And he is a lovable goofus. So, him losing that inherent silliness would be creepy. Finally seeing him get serious during a fight would be a memorable moment. Plus, he's a skeleton. So, a "devil" version of him would just look cool.
And there's his connection with Gunko. Which would be a good reason why he would be singled out to get Reversi'd.
So, yeah. If only one member of the Strawhats ends up getting Reversi'd, I think I'd like it to be Brook. Plus, he is kind of a sitting duck right now, along with everyone else. I don't know if he's the most likely. I'm not sure if any of the Strawhats can get Reversi'd on Elbaph, if the power is tied to where people's "homeland" is. But, he would be the most interesting pick, I think.