Favourite: I have a lot, but the top of the list has to be Hurricane Fluttershy. I liked it the most because 1. Fluttershy episodes are my favourites usually, 2. I can relate to most of the episode with many things, and I personally love it when I can relate to an episode.
Least favourite: Baby Cakes or Cutie Pox. They just seemed boring to me, they're the only episodes I haven't rewatched more than twice :/