For me it's a toss-up between when Gruffi Gummi (with a spell from Zummi) turned from super polite when he heard "Please" to sadistic when he was told "Thank you" ("Sweet & Sour Gruffi"), and when Bubba Duck turned into a genius thanks to a gizmo from Gyro ("Bubba's Big Brainstorm"). It's neat seeing how Gruffi would be if he was nice all the time. And it's entertaining to see a Gummi Bear turn into a destructive psycho. Of course, there was no replacing the old Gruffi. Genius Bubba Duck was such a turnaround from regular Bubba; Frank Welker performed his intelligent voice marvelously. I was close to wishing Bubba would stay like that, but I knew overall it was for the better that he changed back, especially since he forgot his survival instinct.