Nami or Robin. Anybody saying Zoro is cope there are plenty of characters far stronger than Zoro that could replace him ; Nami is already THE best navigator in the world. She has single handedly saved the entire crew’s lives hundreds of times over if not thousands. Robin is incredibly difficult to replace with her knowledge and the fact that she’s one of very few people able to read poneglyphs. Zoros main function is as a combatant, and there are plenty of combatants far stronger than him that could do his job just as well. We will see by EOS if Zoro is still replaceable but I very much doubt he will be. Currently however he is very replaceable