Sanji's. He already beat the guy so both go in with something to prove. Killingham to beat Sanji in round 2 for the sake of the mission and Sanji to unlock COC+ACOC to keep Killingham down to protect the people of Elbaph. I want to see Sanji pushed to the brink, but we'll see. Zaza sounds very strong and flooding a giant village means we could get some large-scale heat attacks from Sanji. Plus, Killingham has a trident, I would like to see him use it. Kata had one and it was cool till he stopped using it after taking a chunk out of Luffy. Seeing Killingham use armament on his and how Sanji matches up would be nice. Also, we've seen Hells Memories twice and Ifrit Jambe Hells Memories once, so maybe we could see the latter again but just like Wano, it could be clear Sanji needs a new finisher (especially after already using Concassé in round 1). A new and unique finisher once Sanji had imbued CoC in his kicks is something I'm excited for. Plus, I'm always looking forward to Sanji fights and to see what new stuff he has (updated attacks or completely new, all cool to me).
I think Zoro vs Super Sommers could be good, but I'm not as excited for it as Sanji's. If Super Sommers has awakening, that could be nice to see, but Super Sommers is the one I want to see tons from. I expect Zoro to do well, so if Super Sommers show off some cool usages of his fruit and can push Zoro far to the point he refines his ACoC then that'll be something. I usually like Zoro fights when they're extended ones so I hope we see more of this than Zoro vs Lucci (there's less going on this arc so I expect it'll be closer to Zoro vs King in focus). Plus, it's always cool to see tons of strong attacks from Zoro and with the focus on ACoC, we'll probably get to see some more of Zoro with King of Hell.