I've been wondering lately about just what it is people enjoy about the Pokémon anime series. I've heard people complain about the 'filler' episodes, which I've never really understood, because this show doesn't really have a single plot, so, like 90% of the show is filler. I know what I personally like about the show, but I want to know if it's the same for everyone else. For me, my favorite part of the series is the personal relationships between characters and between their Pokémon, as well as character development. This is why one of my favorite episodes is the one where Chimchar tries to get over his trauma and adjust to living with Ash. And it's why Sun and Moon is probably my favorite of the series, despite it's unpopularity. The entire focus is around personal growth and relationships, and they do it mostly without battling. And it tackles topics that are surprisingly mature for a show made for kids, especially when it's a show that's been on for so long.
For me, I like seeing episodes with new people and Pokémon, so I can see who Ash and co. might befriend/see later, and also with Pokémon so I can try and guess if it’ll become one of their Pokémon (and inevitably not). I do like character relationships, because it makes them more 3D and less shallow, but I’d have to say that I prefer this type of episode.
I guess those where everyone is split up and forced to work with others they aren't that comfortable with, like AG022 (A Hole Lotta Trouble) or XY071 (So You're Having a Bad Day) Seriously, those are hillarious~!
Honestly, I don’t think I can pick just one- I like them all. This is mostly because I’m not a picky person when it comes to what episodes to watch. I could watch an evil scheme from Cyrus and Team Galactic, or watch Lillie and Snowy bond. I’ll watch whatever and be fine with it.
I'm going to have to agree with a lot of the people here and say episodes with more focus on characterization and stuff. The Pokémon series seems to have developed a theme about how life is about the journey, not the destination. Ash never wins the local tournament (except that one time he did, but no one talks about that) but that doesn't matter because its the experiences he has and the bonds he forms along the way that are truly important. Thus, the episodes that focus more on those experiences and bonds are much more important, and better, than episodes focused more on the league.