9. A Riveting Rivalry
To be honest, there are almost too many episodes from the XY run that could take this spot. A massive leap up in quality for storytelling but also animation and battles after the disastrous Black & White series, nothing epitomized this more than any battle Ash's Greninja got itself in. But it has to be Ash's Pokemon League semi-finals encounter with Sawyer that takes the crown.
XY&Z did a fantastic job of painting Ash as a great trainer, one who used his bond with Pokemon to bring out their strengths, showing how he had grown since his early days in the Indigo League. Sawyer was introduced as a beginning trainer, far more book-study than Ketchum ever was. But even as he develops, he never catches up to the veteran, and Mega Sceptile's clash with Ash-Greninja shows how far Ash has come.
Aside from having some of the best cinematography in the series ever, with fluid and dynamic action aplenty, the battle highlights all the character that's gone into Ash. His confidence is unwavering as he calmly commands Greninja to victory, while rookie Sawyer slips into tears as his offense fails. Its a shining example of character work enhancing action, and shows Ash-Geninjas work at its finest.
And it's best watched in original Japanese, purely for the pumping 'Let'Z Go' theme that accompanies the battle's climax.