Just to help out...
"Island of the Giant Pokemon." (There's subtitles for what the Pokemon are saying, Rukia, even in the dub.)
That said, to answer the question, I think I'll have to pin it on the following episodes, in no particular order:
1. "Pokemon Scent-sation," just for Ash in a dress.
2. "The Ghost of Maiden's Peak," just because of the storyline. (Mmm. Mythology and a non-cookie cutter plotline...) That and it's the first episode I've ever seen.
3. All three Sabrina episodes ("Abra and the Psychic Showdown," "The Tower of Terror," and "Haunter vs. Kadabra"). First off, I always got some sort of sadistic amusement from watching Ash fly around as a ghost. Second, unlike other Gym matches, this is the first time that Ash actually had so much of a difficult time with a Gym Leader that he had to go out and train to face her again. Third, they're the sources of serious gold. Ash dancing with his pants down. Misty and Brock as dolls. Sabrina going all psychopath on Ash when she demanded him to send out a Pokemon... Some of the best moments in the anime right there. Other than Ash in a dress.
4. "Holy Matrimony," just because it reveals a ton about James and gives the audience a glimpse of James as a good guy. It gave the viewers a refreshing look at the bumbling villain.
5. "Princess vs. Princess," probably because it's a festival and competition dedicated to girls. And that rocks.