May. Her team is full of powerful evolved Pokémon, and even the ones that aren't fully evolved are really strong. She also has the best type coverage.
Munchlax? Skitty? Beautifly?
And we do not know for sure how strong Mays Venusaur are. It may be weak and it may be powerful. We have only seen Glaceon in one-two battles since it evolved and no one of them were very impressive. What is left then? Blaziken and Wartortle. Blaziken is powerful and Wartortle too. But that is not enough to defeat Dawn.
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Buneary or Pachirisu did.
Glaceon showed that it was much stronger then Dawn's Piplup who is behind Mamoswine as her strongest. Even then Mamoswine is not impressive in the slightest it has no good wins despite how early it evolved and the only real battle it had it got pummeled. May's team has been gone for 2 generations now and despite that overall she has a much stronger aura then Dawn has ever had. That is hypocritical to say that Glaceon isn't impressive despite showing it's strength yet say Wartortle is strong when all it did was a appeal round. May has a better overall team and more coverage which obviously shows she has a better team then Dawn's.
May, hands down. Her team was more diverse, covered more weak points, had greater balance, and a much smoother and focused style. Dawn's was good, too, but she was more flashy than anything. May, on the other hand, despite her initial lack of confidence at the start of her run, shown to have greater competence, the ability to learn from past mistakes more easily, and more of an ability to improvise on the fly (Combusken's first contest is a good example, able to make a bad situation into a good one). She also never had a Pokemon disobey her once, something that both Dawn and Iris had problems with (Mamoswine, Piplup at the start, and 3/4th Iris' team disobeyed their Trainers at one point or another).
So May had the best team of all the companion Trainers. And she herself proved to be a very good Trainer, even though she had a rocky start, something that her Pokemon reflected.
Best Game Characters of all Time: Ryu 3, Nina 3, Ryu 2, Nina 2, Rei, Ratchet, Talwyn, Princess Pride, Megaman.EXE, Elecman.EXE, Bass. EXE, Tails, Geo, Sonia, Pit
Best Anime Characters of all Time: Van, Fiona, Irvine, Cynthia, Yugi, Tea, Joey, Dark Magician Girl, Kuriboh, Yuma, Astral, Number 39: Utopia, May, Inuyasha, Miroku, Sango
I apply Felix's Law: "Smile, tomorrow could be worse."
With DW abilities: May
W/Out them: Misty
I was going to say Brock but reading the topic changed my mind to May.
I always liked Dawn's team for some reason. It had evolved Pokemon and un-evolved Pokemon. It just seemed really balanced.