Here's my top 15, although there are so many more
- Ash beating Leon with all the companions and mons just HAS to be no. 1
- "Champion", "Champion", "Champion". Legit easily the best moment ever before Ash vs Leon, what an epic way to cement Ash's alola champion status and continue that ever since in every Masters 8 episode
- Ash, Dawn, Brock, Goh and Cynthia taking on TG once again. Those legends Arceus specials were just DP reborn man, they were too perfect
- Lillie's episode, the entire story, the way they reunite, it's beautiful storyboarding, it's such a perfect episode
- Ash's Riolu and Goh's raboot evolving into Lucario and Cinderace and then the whole aftermath. The whole DD arc in general in the anime was really top tier albeit rushed
- Paul's return episode, all the Oak mons interacting with JN mons, it was beautiful to see
- Ash beating Cynthia. The slugfest between Lucario and Garchomp was BRUTUAL
- Lei being introduced and the first Alola episode in general
- Serena's return episode, they handled her character so well
- Goh and Drizzile looking at the starry sky and the conversation they had, and it coming full circle when Inteleon evolved and looked at the night sky along with Goh
- Gary's return episode and seeing Infernape again
- Iris's return episode and seeing her battle Ash
- Ash vs Volkner
- Project Mew's finale with the Regis and Goh and Gary
- Ash catching Dragonite, Gengar and Lucario
- Sirfetchd's entire arc