I actually think it's Dawn. Not just because she and he are really really close either, though that helps a ton. It's because Dawn was with Ash the most she could be. Even despite her own goals and dreams, she somehow made journeying with Ash her thing. Other party members came and went with regularity in Sinnoh; but Dawn rarely ever split up and took a different path than Ash did; and only when necessary.
Granted that alone, Dawn probably would have been in lots of trouble; despite her brilliance as a coordinator; she really wasn't all that great at battling much until Ash got to know her and taught her a few things.
While Brock has always "Just been there" for Ash as much as he could; he's kind of like an older brother. Yes he contributes to Ash's growth; but not directly, ever, the way Dawn does by being Ash's 'student' in plenty of ways. Brock is good at making sure Ash isn't lacking in survival skills. But that's all Brock does. He never really spends a whole lot of time teaching Ash how to be a better trainer; instead he seems to expect Ash to figure it out. I suspect that's just the Gym Leader streak in him. Brock always seems like he'd rather just be there to impartially officiate Ash's Battles than to do much else. Of course he also spends a lot of time chasing skirts, the dirty boy. It seems to fall on the "Female Companion" of the generation or moment to give Brock the "Down Boy!" treatment. I can't fault Brock for his Loyalty to Ash but I also know his motives aren't just for Ash nearly as much as they are for himself. Ash endears himself to many, many ladies. Brock enjoys this. A little too much sometimes. ;)