I still think the worst overall episode of the classic era is The Canine Mutiny, as, unlike the small number of other classic era episodes I don't care for, it has virtually nothing to offer.
>>112801013 mentioned Bart's Inner Child, and that's a good example because although that episode is completely void of substance in terms of plot, I'd say there are still plenty of good gags present for it to be enjoyable and rewatchable. In TCM, however, not only is the story dull and unmemorable, but there are only about five decent jokes to compensate for it.
Bart Gets an Elephant is another glaring example for me--extremely obnoxious in every way.
