None of them have Arbok and Dustox in the same episode...
No, but seriously, my favorite would have to be
The Art of Pokemon. It really made me remember my love for artistic pursuits. Not necessarily visual art, but moreso in areas I can actually do okay in (writing and voice acting, to be exact).
Plus, I discovered the art of Jackson Pollock, and I love it. I dunno, it was one of those episodes that not only had the entertainment factor, but actually had meaning for me. Funny thing is, I never actually saw it until recently.