Going back to the topic what what do you think can be a good reason on why ash would romantically prefer Serena over the likes of Dawn and Misty?
To refrain from talking about other ships as much as possible:
Honestly, there is no correct answer.
As I said earlier, Amourshipping was great with XY Ash, because XY Ash was much more of a knight in shining armor. He was overly enthusiastic and "childish" at times, but he was mostly upbeat, polite, encouraging, "mature", and obviously heroic. That meshed well with Serena, who looked up to him, and almost never bantered with him or got under his skin teasingly. That's not to say she wasn't ever afraid to call him out (the snowball scene), but the basis of their friendship/relationship was being very supportive of each other. It's healthy, but it doesn't bring out certain aspects of Ash's character (see below). I'm sure JN Ash, who's basically an amalgamation of previous Ash personalities, would act similarly with Serena if she returned for an extended period.
However, compare that to the JN Ash that we saw in the most recent episode. Misty and Ash just have a different type of chemistry. Some might prefer the comedic banter, saying that's the basis of flirting and mutual attraction. Others would call it akin to quarreling siblings (though I think this description is closer to Ash and Iris, who have a slightly different dynamic), and prefer Amourshipping's vibe. I will say that I missed sassy Ash. I think the sassy Serena that we saw very early in XY when walking with her mom could have been perfect to bring out that side of Ash too, but that didn't happen.
Dawn has a friendship with Ash that's sort of in-between that of Misty's and Serena's, though maybe closer to the latter. They both are also very supportive of each other, but they aren't afraid to bicker and tease one another. It could arguably be the best of both worlds. However, their relationship is so platonic, and one could argue that they're practically twins.
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Regardless, there doesn't have to be a good reason. Amour is canon from Serena's side. If nothing else changes in the status quo, she 'wins' by default.