After Episode 1 and 2, I thought Serena would be sassy and welcomed the future banter with Ash. Especially after the heavy, early ship tease. Instead, Serena ended up getting relegated to the sidelines for much of year one, and was honestly fairly boring. She didn't have much presence outside of her focus episodes, and that's still an issue, though to a lesser extent, to this day.
Episode 60 was one of the best episodes in the series, and I thought her goal arc would pick up after that platform. However, Showcases became a more boring Contest - somehow the less exciting bit of Contests (Appeals) ended up being the more exciting bit of Showcases (Free Performances). That should tell you enough.
And this post from several months back (before the Master Class) still remains largely true:
Might as well give her the Best Waifu Crown, since that's how she has been portrayed in XY: as a pretty, polite, and nice girl whose faults are endearing (not wanting to get dirty, getting jealous over Ash). An excellent fashionista, cook, and dancer, who can be a badass Rhyhorn Rider when necessary. Everything has come so easily for her in these Showcases. All the themes were conveniently stuff she's familiar with. Fashion/dressing up, Pokepuff baking, quiz question about Sylveon that she has seen twice already, and a stable of Rhyhorn. She has the looks and is a talented dancer, which are big in the Free Performances. Her Pokemon were meant for this, and naturals at making combos. Her two losses were flukes, otherwise she probably could have won every single Showcase.
She's basically depicted as top waifu material and love interest for Ash. Unfortunately, these writers seem to have forgotten how to write semi-decent romantic subplots from back in the day with Ash/Misty and May/Drew. They rushed to status quo by episode 7. Even after Millefui 'threatened' Serena in episode 26, she still hasn't made any moves. Everything from Ash's side can be just-as-easily interpreted as him being appreciative and kind, as a friend. At this point, if they decide to 'canonize' SatoSere, it's going to be forced no matter what. Oh yeah, you can make a drinking game out of the number of times she's been fixated on "Never Give Up Till the End."
Despite the flaws, Serena is still a likeable character and I wouldn't mind if she went along to Alola as long as she did something other than Waifu Wars. She definitely has some emotional depth to her character. The river scene during the camp arc and the snowball scene with Ash were some of the best genuine human interactions in XY. Her Pokémon are likeable, though a bit overpowered.
I'd say she ranks:
1. May
2. Misty = Dawn
3. Serena
4. Iris