I cannot pick out a UNIQUE "worst episode ever" of The Simpsons. Perhaps there is none, even by ANYone's standards. There certainly are episodes that I do not care to see again. But, even those episodes were funny the first time I saw them. I just know they will not be funny the second time.
However, even those episodes which I found a bit boring -- I would not say ANY of them is a "piece of shit". Even they had moments throughout of funny original gags and jokes.