Best;
Ahh, geeze. This one is difficult, because there are so many that I enjoy. 'Ash catches a Pokémon' is a personal favourite of mine, though I'm sure it's not the best one out there. 'Claydoll, Big and Tall' gets my vote I suppose. Excellent animation, hillarious expressions, a truely bizzare storyline and a ton of memorable scenes made this one worth watching. If anything can ever top the concept of seducing a giant slab of living clay with a cheerleading cat (outgoing poisonallitay), disturbingly apparant jailbait, blobs that pass as fanservice and a ballet-dancing prima donna, then I'll be downright shocked.
...ahh, the hell with it. 'Ash Catches a Pokémon', 'The Heartbreak of Brock' and 'A Better Pill to Swallow' are all just as good as 'Claydol, Big and Tall'. Hell, 'A Better Pill to Swallow' may even surpass it.
Worst;
Late Houen's Huntail and Gorebyss filler. Honestly, the entire thing was trash in it's purest form, and I'm really having a hard time remembering a Johto episode that was as poorly written as this piece of yawn inducing faecal matter. I couldn't give a damn if they were using it as a way to show off a branched evolution, there was absolutely no excuse for how mind-numbingly dull they made it.
The fact that Huntail and Gorebyss' only episode had them swimming around in circles whilst slapped into some random pool the entire time doesn't exactly boost it to the top of the charts either.