I honestly think it will be Killer. The thing is, in sabaody the supernova were roughly grouped in 3 separate ways.
You have capone and bonney, who only took out fodder, aren't really fighters, and have high bounties for other reasons.
You have the mid tier supernova: Drake, Apoo, Hawkins, and Urouge, who are good fighters but don't have any major significance beyond that. These 5 all fought the pacifista and kizaru together
And then finally you have the rooftop 5. These 5; Luffy, Law, Kid, Killer, and Zoro all saw Luffy punch the celestial dragon, all saw Rayleigh use haoshoku haki, and fought alongside each other to escape sabaody. They were also the main fighters to take on big mom and kaido.
The rooftop 5, out of all 11 of the supernova, are the only ones portrayed to be the leaders of the next gen. They are the only ones who are hyped up to be future legends. And this includes Killer.
Killer is one among 2 supernova who aren't captains, the other being zoro. Despite him being significantly weaker than zoro, he is consistently portrayed as zoro's rival, imo in ways sanji just isn't. Ultimately, it was Killer, and not sanji, who fought alongside zoro against kaido and big mom.
In the old gen, there are only right hands who became legendary, despite never being captains. Those being Rayleigh and Oden(who was whitebeard's right hand). imo, Zoro and Killer are a direct parallel to this.
Given how all of the rooftop 5 witnessed Rayleigh use CoC in sabaody, and how 3 out of 5 of them already have CoC, I'm inclined to believe that by the EoS, Law and Killer will both have CoC.
Sanji might get it, but honestly, I think Killer could get it first. At the very least, I doubt Sanji will get it before Law. At the moment, he's not fighting anyone who I think really requires him to have CoC. I think in the future if he fights an admiral, he could unlock it then.