Spike at Your Service is the one of the most generic story FIM has ever done, Crystal Empire was a little disappointing, Canterlot Wedding is still pretty popular thanks to Chrysalis and the Changeling Zerg Rush, Wonderbolts Academy is MLP: Top Gun(all that was missing is Danger Zone), Bridle Gossip was a little bit about racism, Griffon the Brush-off was about a Fluttershy getting bullied, Mysterious Mare-Do-Well was consider the worst episode of season 2 but it's better than quite a few episodes in season 3, and Equestria Girls should have been just an hour long TV special and not an hour and a half film in theaters.

The Canterlot Wedding is actually overrated.

The writers could have done WAYYYYYYY better with this, this was just a filler cause there is a big special at the beginning of each season and at the end. They made Sombra up and then killed him so Cadence could have a kingdom and give her more screening time. Unlike with all the new characters they add in, they never made him important like with Candace, Shining Armor and Discord. Truly disappointed.

I'd say the Magical Mystery Cure, because of the whole alicorn Twilight thing. Besides that, it was a descent episode with good musical numbers.

Sombra died. And this is a kids show?

@karinabrony, indeed. I also have to say, The Crystal Empire is my favorite episode and I don't know why it's so underrated... By the way, the question was 'underrated episode', not 'movie'.

MLP: EG is NOT an episode! It's a movie! Canterlot Wedding was a little crazy, even though I never saw it.

Mysterious Mare do Well was about RD's friends being better than her, not a very good lesson learned there, Twilight...

i dont like spike at your service as much