Zoro could sometime replace Luffy in fighting. Unlike Sanji, I would expect Zoro to keep up with Doflamingo if Luffy is not there.
Robin is a key to one piece.
Nami help navigate. I would expect she play a major role to reach destination even if it is off panel.
Sanji, a gag character. The crew could survive without his cooking. For the fight, we already have Luffy, Zoro, Jinbe that's above him.
So I would vote sanji