Too many I would guess but probably the very 1st episode. I honestly have to say that because it was so... I guess "Exciting" to see what I did in the game in anime form. Of course not 100% accurate until Yellow Version came out, but I thought it was really cool. Led me to continue watching until they introduced May.
GT: Ayumi Spender || PS0: 4597 9585 4793 Only talk to me when I order you to. 3DS: 2921 9091 2567
The episode in Sinnoh with the Evil Togepi. I swear, it was like crossing Pokemon with the best of classic Looney Tunes. It was hilarious.
Wow, someone else said this! Came here to say the exact same thing, that evil Togepi was awesome!
I second that. Evil Togepi was awesome.
Other episodes I really liked(most are from season 1):
The one where Scyther and Electabuzz fight and Pikachu falls in love with a bottle of Ketchup(I think it was Duel in Dark City?) The episode in which Charmeleon evolves into Charizard to save Ash from the rampant Aerodactyl.(tough I like most Charizard episodes^^") The episode which featured the battle between Pikachu and Bellsprout on Indigo Plateau. The entire battle was really cool.
EDIT: Forgot one. The episode on the island with the giant robo pokemon. It was the first episode to translate pokemon speech via subtitles iirc ans Squirtle was pretty funny in that one.