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Dawn > May > Iris > Misty > Serena
Seriously, Serena is incredibly fucking unengaging as a character. She's just there so that Ash can push her away. There's nothing that you really want to know about her, since nothing about her is interesting in the slightest. The entire extent of her character is built around Ash, and having no goal introduced this far in really makes it hard to believe she'll ever stop being the absolute worst Pokegirl.
Iris was annoying as fuck at times, but at least the episodes that focused on her were enjoyable. The fact that she displayed no romantic feelings towards Ash, and was just a travelling companion made for a nice atmosphere. Also she's cute as fuck.
Misty went from kind of annoying to just boring, though I still don't really hate her.
May was really enjoyable, and developed goals and shit. She's the only character I could actually see together with Ash, it felt like they introduced Drew only as a reason for her not to fall for Ash.
Dawn was the absolute best. She developed the most during their entire journey, and all of her ups and downs genuinely made you want to see how things were going to end up for her. She felt like a great bro, and them actually displaying that she and Ash were unable to actually display romantic emotions because of their bonds as friends. Her bonds with Piplup felt stronger than the ones the other Pokegirls had towards their Pokemon.
Also, her winter outfit is the best outfit.