It's pretty much impossible for me to pick a single episode, especially from the earliest seasons. I like SO many of the Indigo League, Orange Islands, and Johto episodes. One-off episodes like "Mystery at the Lighthouse" or "The Ghost of Maiden's Peak" were always fun and made Pokemon feel magical and mysterious. "The Double Trouble Header" definitely played a non-zero part in which NPB team I root for, since Casey's Electabuzz baseball team is clearly modeled after the real life Hanshin Tigers.
"Hello, Pummelo!"/"Enter the Dragonite" probably make up my favorite battle. I like how powerful Dragonite seemed and it's a big part of why I like Dragonite so much. I also really enjoyed the Ash vs Gary showdown in the Johto Conference, in episodes "The Ties That Bind" and "Can't Beat the Heat!"
As for "newer" series, I really loved "A Showcase Debut!" from the X/Y series. I like Serena's resolve and determination in the face of failure, paired with the support her Pokemon give her.
