Sanji favored Luffy (not that he'd ever admit it).
It was hard to tell, but the little things that he did concerning his captain, the ones that nobody seemed to notice, were the actions that confirmed it.
For example, when it came to choosing what to eat, Sanji always asked Luffy first despite knowing his captain's answer. And despite his threats to confine Luffy to a vegetarian diet, the chef always made sure that the majority of food he bought and cooked was high quality meat. Not just high quality meat, but the kind that made Luffy smile the widest (although not many people could tell, since Luffy seemed to be a fan of all meat in general).
And despite his constantly grumbling about how making food for Luffy was too much work, Sanji always had extra servings for the bottomless pit that he called his captain, and refused to slack off on the quality of the taste.
The others seemed to expect him to favor the ladies more, but when Sanji made sweets for Nami and Robin, he always had an extra serving (or two) for his captain. The blond excused it by saying that he just didn't want his captain to eat the food that belonged to the ladies, but he always made an extra large portion that was specifically made for Luffy's monstrous appetite.
But it was easiest to tell when he fought.
He didn't just fight for the adrenaline rush... although that might have been part of it. He fought because when they won, Luffy smiled really widely, like everything was alright. And though he'd never admit it, that comforted Sanji.
Because when Luffy smiled, everyone else on their crew did too.
Sanji favored Luffy, but nobody else ever commented on it.
Because maybe everyone else favored Luffy too... not that any of them would ever admit it.
A/N: I may also do a oneshot for the other crew members too. I think it would be fun to write Zoro and Brook... but maybe it's just me. I dunno. Anyways, I hope you enjoyed this.