Bond with Ash:
1) Serena
2) Dawn
3) Misty
4) May
5) Iris
Ash and Serena's bond isn't that obvious initially since she was pretty shy and quiet. However, it was clear that emotionally, they really helped each other through different odds, such as the talk when they fell off the cliff, Ash remembering her advice for the Korrina gym battle, the meaning of a simple ribbon for Serena, and her crucial role in helping him snap out of his issues in the winding woods.
Dawn and Misty are almost tied (though I give a slight edge to Dawn), since they have the most visible friendship and bond with Ash. Dawn and Ash were equals, and their constant training together, hi fives, cheerleading and similarities in attitudes really showed they were always on the same wavelength. Misty was his original friend, and along with Brock, was crucial in shaping Ash during his early years where he was a hot headed, cocky brat. Beneath her fiery exterior, Misty showed plenty of moments where she feared for Ash during any danger, and her care for him.
Goal Improvement:
1) May
2) Dawn
3) Serena
4) Misty
5) Iris
Dawn's contests looked better in DP for sure, but I still think May's constant improvement in contests was more engaging as a viewer. Unlike Dawn, May didn't know anything about contests, and had to constantly improve by herself. She also became super vain, and got a rude reality check. Also not to mention, she had better rivals in Drew and Harley that really pushed her.
Looks:
1) Serena (after XY060)
2) May
3) Misty
4) Serena (before XY060)
5) Iris
6) Dawn
Now this is the most subjective category, but I really loved Serena's makeover. Her short hair really suited her, and looked much classier, along with giving her a more matured look. Equally close is May's, as I liked her bandanna as well, and overall color theme of red and black.
Pokémon:
1) Dawn
2) May
3) Serena
4) Iris
5) Misty
Dawn's Pokémon suited her perfectly. They all had their own character and each member had their time to shine in contests. From the proud and cheery Piplup, hyperactive Pachirisu, stubborn Mamoswine, and even the last minute Togekiss, which showed its caring nature, all her Pokémon had various personalities and were really fun.
Now based on just these categories, Dawn edges out Serena for me, but if it included character progression and development as well, Serena would've been ahead of Dawn slightly, since that's the primary reason I like her.