A lot of people are picking episodes like Move Along Home, If Wishes Were Horses, and Fascination.
To me, episodes like that can’t qualify as “worst” because they don’t take themselves very seriously. If the attempts at whimsy fail, then the episode is at worst a forgettable waste of an hour. An episode like Melora, on the other hand, is memorably bad.
Ah, but the question in the thread was, "What is your least favorite?" not "What is the worst?" From my point of view, it doesn't really matter to me whether an episode is supposed to be taken seriously or not. Bad is bad, and failed whimsy can be just as painful to watch as failed epic plot. I suppose the only way failed epic plot might edge out failed whimsy for me is that with failed epic plot, it can drag down an entire arc rather than just serving as its own little corner of stand-alone Purgatory.