Sorry I can't pick one scene there are a bunch of Sanji scenes, but tbf they all came at the same time during one arc (WCI).
-Beating the shit out of his captain
-Trusting BM's word (his enemy) like a tool instead of consulting with his captain and crew
-Crying outside a 16 year old's bedroom
-Risking his Captain's/crewmates lives to go save his rapist, murdering, siblings' lives
-Baking his sworn enemy a tasty cake while ignoring his crewmates who were gonna die
As you can see Oda did his best to assassinate one of the original Strawhats. The character will never be the same.