Tentacool & Tentacruel. (Episode 19 of the Original Series)
1) The story actually has a decent moral: If you mess with nature, it will come back to bite you in the ass.
2) Some of the weakest pokemon are shown to be capable of overthrowing mankind and destroying civilization, if provoked.
3) Misty begging the Tentacruel to stop destroying the city(/world?) is one of the best scenes ever.
My favorite would be The Bicker the Better!
Team Rocket made me laugh :D
I don't have a single favourite episode as I enjoy all the episodes so I have many favourites.