Going to mend 28 Pranks Later even more now that One Bad Apple is starting to be ruled out from being the worst, as in Mare-Do-Well that the other ponies are that indirect and that bad friends about emotionally wrecking her for something as minor as Rainbow Dash bragging, without even telling her just bothers me far more than the other two episodes. 28 Pranks Later at least tries to have a better "eye for an eye, tooth for a tooth" justice system even though it doesn't do it so well.
The Mysterious Mare-Do-Well: -36
One Bad Apple: -10 28 Pranks Later: -22
Interesting that these are the two contenders considering that 28 Pranks Later has a similar plot to Mare-Do-Well, but even then Mare-Do-Well is still the worse episode as I see it because Twilight, Rarity, Pinkie Pie, Applejack, and Fluttershy are portrayed as heroes for not even dropping a hint that Rainbow Dash is bragging beforehand and then emotionally wrecking her by one-upping her as Mare-Do-Well, plus then they're hypocrites and brag about their accomplishments.
The Mysterious Mare-Do-Well: -36 28 Pranks Later: -8
Ah, now we have... the worst cartoon episode. The Mysterious Mare-Do Well didn't do so well here. Dishonor upon you, Mare-Do-Well. The Mysterious Mare-Do-Well: -36 28 Pranks Later