Summary

• Ash's victory over mentor Cynthia signified more growth than winning the World Coronation Series against Leon.
• Cynthia's mentorship shaped Ash more deeply than his rivalry with Leon, leading to his ultimate triumph.
• The battle with Cynthia was a culmination of Ash's journey and a true testament to his development over time.

While Ash's climactic Pokémon World Coronation Series battle against Leon may have been his most impactful victory in-universe, but it wasn't the most significant for his character. From a storytelling perspective, Ash's victory against his long-time mentor Cynthia was actually a much bigger milestone.

Leon functions as a rival for Ash, one who's only just entered Ash's awareness when they first meet, and Ash wants to beat Leon so badly specifically because no one has ever done it. While Ash and Leon do develop a bit of a relationship over the course of the series, working together to deal with Eternatus and so on, it's not hard to argue that Ash and Leon's relationship isn't a particularly deep one.

That's especially true when compared to other Pokémon champions, like Lance, Iris, and Cynthia, whom he's worked with and learned from for a much longer time than Leon.

Ash's Battle with Cynthia Better Realized His Growth

Cynthia first appeared in the Diamond and Pearl episode "Top Down Training!", where Ash and his rival Paul rushed off to meet the champion upon hearing she was nearby. Paul was easily defeated when he challenged Cynthia, but Cynthia turned it into a learning experience for both Ash and Paul, igniting a desire in Ash to battle and defeat her some day. Cynthia would go on to appear more throughout Diamond and Pearl, helping Ash to confront Team Galactic and put a stop to their plans, all the while giving advice and helping Ash grow.

Cynthia goes on to appear in Black & White as well, this time guiding not only Ash, but Iris as well. Cynthia's role is much more mentor-like throughout Black & White, with Cynthia having seen Ash in battle at the Sinnoh League championship, commenting on how she'd like to battle him when he fully matured as a trainer. Cynthia does what she can to help Ash to mature in that regard, and is more than happy to help his friends and even rivals as well.

This makes Cynthia's return in Journeys all the more significant; the day has finally come when Cynthia is meeting that more mature Ash on the field of battle. The fact that he's even standing there shows how much he's grown since she first met him, and how he has gone on to live up to the potential she saw in him. Cynthia is even shown having doubts about her own future at this point, considering retiring from being champion before her battle with Ash, but seeing how far he's come with her own eyes and experiencing the intensity of their battle, she decides not to.

Ash defeating Cynthia in the World Coronation Series was the culmination of a goal that he began working towards over 600 episodes earlier, and ultimately acts as a better signifier of his growth over that time than winning the title of Monarch does. Cynthia was a far more major influence on Ash than Leon was, and finally being able to challenge and defeat her proved once and for all that Ash had grown from the days of Pokémon Diamond and Pearl.