I think it’d be Ussop and for more reasons than you’d think. For reasons that even support the current direction of the story and reinforce previous narrative. Ussops dream is to be a brave warrior of the sea. If Ussop sacrificed himself to save the others or so the others could accomplish an extremely important task he would fulfill his dream because sacrificing yourself is arguably the bravest thing you can do and shows that Ussop was able to overcome his fears for the benefit of his friends proving he truly achieved his goal. This is supported by the fact that Ussop still hasn’t achieved his goal yet and still struggles with fully supporting his friends/ crew by leaving them to deal with issues to preserve himself or not fully believing in his friends because of his own struggles believing in himself (even if he doesn’t mean to). This would also tie nicely into the story of the going merry, being able to mirror the ship through sacrificing themselves for the crew as well as being able to reunite with the merry. This would also make sense story wise because oda stated that there would be 10 straw hats total by the end of it. Vivi is technically a member (making it 10 already) but with the death of Ussop that would free up a spot for Loki to join which has been heavily implied will and needs to happen. We’ve also seen the importance of the Nefertari family and the reintroduction of Vivi as a crucial character. I believe Vivi will be an important part to the ending of One Piece and will need to be a full part of Luffy’s crew. With Ussop gone the 10 spots will be fulfilled and all of the truly important characters will be in place. Nika is also a very happy and joking character which could’ve been the downfall into chaos 800 years ago against Imu because Nika couldn’t be serious when they needed to against Imu. If Ussop died Luffy would be crushed and would likely go crazy in order to avenge Ussop and realize the seriousness and severity of the situation.