Easily Dawn, she had the best and most natural interaction with Ash, both are like perfect pair whether you look at them as couple or siblings, had tag team for many battles and most importantly how they help each others to grow which is something Dawn did better than every other companions (maybe close to Brock), their iconic high five, there are many more things I can say about this duo.
I will add my another favourite Gladion here as a rival, after Gary Gladion is the closest friend to Ash among rivals, Gladion is the only human character in entire anime whom Ash spend entire episode alone in SM47, both of them influence each others to get stronger, the impact Ash had on Gladion is bigger than any other rival, their battle style and even all their Pokemons during Alola league finals actually so similar, Gladion is like edgy version of Ash.