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Serena is the best girl for Ash because she is the one that has his mentality but for an activity that is completely separate from his own. All the other girls are battlers to a greater or lesser extent, which makes them all possible obstacles for his quest to be the very best at battling. He can't support them 100% because at one point he'll have to beat them or be beat by them.
Serena has this "be the greatest" mentality but for performances, which involve no battling at all. Which means she's the only girl who has a trait he likes (determination and ambition) but who he can support 100% without compromising his own goal.
