Funniest episode for me, I don't remember exactly what its called, but I think its called flaming moltres or something? The one where James discovers his zodiac is a moltres and puts on that new persona and costume, I've never laughed so much at a cartoon, it was so good.
Saddest, there's actually quite a few honestly. Bye bye butterfree, Gotta catch ya later, Memories are made of bliss, the one where Ash almost lets Pikachu go (can't remember the name), god I cant decide. And that's without even mentioning the movies.