I couldn't even answer that. Collectively, I'd say XY had the best ones, but I'll just say my favorites
1. XY131 or XYZ28 - Character development for Ash?!? What is this?!? We find out why he actually became a trainer and got the closest to the definition of a "Pokémon Master" that we've ever gotten?!? MY WORD!!! Not just Ash?!? We're building on the Ash-Greninja storyline? More development for Serena?!? You're just stuffing this girl with it like she's a Thanksgiving turkey!!
"Serena's too submissive to Ash, she's useless, she has no character development"
Bull and crap.
This girl just stood her ground to the guy she's "too submissive to", and told him off for taking his frustrations out on when she's trying to be the emotional crutch she's consistently been for him. Rather than tear him down even further in a shouting the match like the other girls would/have done in that situation she picked him up and told him what he needed to hear and actually got through that thick skull of his and made him think about his mistakes and pull him out of a depression when her predecessors couldn't even do that. This is the same girl who worshipped the ground he walked on and thought he could do no wrong, and let him run rampant. Now she still have a rather high opinion of him, knows he's fallible but still doesn't look at him any differently, AND can keep him grounded and bring him back from his worst and push him to his best.
Emotional/Dark/Serious/Mature....is this Pokémon?
2. XY140 or XYZ47 - A beautiful wrap up to the XY Series. Really showed the XY Gang as being equals despite Ash and Serena being the stars, showed how great their chemistry is and how close the 4 of them are. Gave EVERYONE some time to reflect and not just Ash.
Clemont - Meeting/Bonding with Ash, his first partner Luxray, growing more confident to be the gym leader he wants to be
Bonnie - All of the Pokémon she met, getting firsthand experience on being a trainer taking care of the group's pokemon.
Serena - Meeting her Pokémon, finding her dream when didn't start with one, earning her mother's blessing, and replaying Ash's words of him always supporting her.
Ash - Thinking about the journey, meeting his pokemon, his rivals, important stuff, his friends (Although his flashbacks regarding them had a heavier emphasis on time spent with Serena alone while the siblings were apart of group moments) Plus the whole Ash/Serena kiss things sent/sends me into an adrenaline filled frenzy of excitement. Less manly? Yeah. Do I care NOPE