Jimbe
For 99% of the story they managed to do just fine without him in the crew. The crew managed to sail with the sunny just fine, they don't especially need a special advisor because they either are lucky enough and go straight to the enemy or befriend their enemy or local people.
Except during Green Room... Doubt Franky would pull that feet off!
So I respectfully disagree
Every crewmember is essential to the crew..
However, I can say that some members are REPLACEABLE, and their position in crew can be compensated by someone else in same field/area
But overall, everyone saved the crew life at one point!
If I had to choose a crew member to be replaced by someone, I would go with Chopper replaced by Law
In fact, the most crucial time for Luffy to be healed by someone was during Impel Down (by Iva) and post-marineford by Law
Which is weird as fuck
Chopper's best moment as a doctor was after Aokiji fight when he was not heating Robin's body fast, rather gradually submerge her in warm water and change tempreture gradually