Show Stoppers. I don't like the Cutie Mark Crusaders to begin with, and that removed what little interest I could have had in them by making it ludicrously obvious what talents of theirs will get them their cutie marks, but having them be too stupid to notice. Also, the song. I only like maybe half of the show's songs, and that one isn't among them.
While I agree that the aesop at the end rubs me the wrong way, aside from that I liked that episode. Plenty of classic cartoon antics and slapstick (which I have a weak spot for), plus the Hydra is one of the few real action scenes in the show, plus, as you say, Rapidash Twilight.
Zevox