Honestly I feel like smoker might be but I think coby is the better choice .[btw is his name spelled koby or coby]Because him and luffy go back ever since the first episode,
Koby of course since he is meant to become fleet admiral while luffy is set up to be pirate king so him and i personally love the contrast between the two
But koby is still too weak at the moment but koby mirrors garp well since he is also considered a hero of the marines although to a lesser extent and he has no devil fruit just like Garp
