So in the longrun, I'd say that Dawn was the one who affected Ash most as a Trainer, while Brock was the one who truly acted as a coach.
Oh, and Iris might not coach Ash, but saying that she doesn't contribute to his growth as a Trainer is wrong too. That you should've noticed. After all, Iris was the one who taught Ash about the basics when he first arrived to Unova. He taught him about the Battle Club, she (and Cilan too for that matter) taught him about the Unova Pokémon, and always pointed out type advantages in battles. She even inspired him to continue battling, even when everything seemed hopeless. No main girl has ever had such a huge effect on Ash before, but Iris had. During his 8th Gym Battle, she and Cilan was the reason to why he actually considered battling, and didn't give up.
Misty definitely coached Ash about the minor basics though, just as Brock did.