I honestly can't fathom why everyone hates the CMC episodes so much. I think they were pretty funny. I mean, sure they don't usually include the mane 6 as much, but come on, some of them are pretty good episodes. I know for a fact at least two of my friends that are bronies not from newgrounds skip every CMC episode.
On the current subject, a worst episode is really hard to pick. Honestly, I haven't re-watched any of the episodes recently so I'll have to go over them. Recollecting the recent ones however, I think that most of the Applejack ones were the worst, especially the one where she runs away for getting 2nd place.