It's hard to narrow it down, but after rewatching so much of the Kanto era on my computer lately, I have to narrow it down to "Ghost Pokémon and the Summer Festival", otherwise known as "The Ghost of Maiden's Peak". This episode is just bursting with stuff you never see on the show anymore: really clever dialogue, outlandish surrealism, a captivating mystery, and - most importantly - plenty of Japanese culture. First of all, it's the initial episode in which Takeshi is revealed to really be brimming with heterosexuality, as he's so depressed over the end of swimsuit season that he's instantly smitten with the phantom maiden. Plus, the episode firmly establishes that Musashi and Kojiru are really at the bottom of the Rocket-dan totem pole, as they have to crawl around the Summer Festival on their hands and knees looking for discarded yen. The music in this episode is superb, too - listen in the Japanese version for the familiar "Pokémon Tower" music from the Red and Blue video games (the tune is replaced in the dub). And the scene towards the end where Ghosu is constantly scaring off everyone's Pokémon with their greatest fears is pure genius (especially when Kasumi holds up a cross in defense and Ghosu matter-of-factly states that he's not a vampire). Great plot, great characterization, great writing, great music, great voice acting, great animation...and Kasumi looks pretty cute in a kimono, too. This episode has it all.
