Let me just reach back 20 years into the archives.... I can't pick just one, so oof.
Kanto, I really liked Electric Shock Showdown as a kid, because I loved Pikachu.
Also Holy Matrimony, because I used to be a Rocketshipper. LOL
for Hoenn, Come What May! and episodes around it were good!
That was when Drew gave May the rose and they had one of their contests that I enjoyed.
I also liked the episode where Ash and Gary kinda fix their friendship/rivalry, and you get to see the little Pokeball that split in half and held on to.
Edit: Google tells me that one is called The Ties That Bind lol
In XYZ I really liked some of the Greninja drama episodes, and the one's where Alain showed, especially!
Ash vs Drake-It was the first time he won a league. How do you do a "Be Continued" during an attack? Was eager to watch the next one.
A close second is The Rival Showdown, or something like that....Whatever it was called, but I believe it was when Gary's Eevee beat Pikachu. It was when Gary changed, & grew up. Ash was still stuck in his own ways, & Gary thrashed him...Beat him down & just left...Ash was super salty.