Luffy, Zoro, and Koby clearly had mentors that had CoC and know they will surpass them. That means they were marked for greatness and must be CoC users. I know people will cope about Koby and some about Mihawk, but its not only what happens in Shounen all the time but its also the writing on the wall. I think many blind themselves to the reality that Oda has setup up these 3 from the beginning of the series for a reason and its insane to think that they won't all be top 3 come EoS. Mentorship in OP has always showcased that is the wall that each one has to surpass, and only Luffy has surpassed Ray but still has to surpass Roger of course due to his overall dream. Mihawk clearly has CoC considering Zoro has it, and Koby clearly has it considering Garp has it. Now as for the whole Aokiji was Garp's disciple and he should have it well its actually likely, I will say that those that have top tier Logia DFs have to train both the DF and their bodies along with haki which means that his CoC abilities might not be as refined as Garp's overall. Kaido did say haki above all after all.