Double episodes and movies count as a single episode.
For me:
Dragonshy
And not just for the obvious reason. Many people cite this as the episode where they officially fell in love with the show. It also had a bigger scale than anything else the show had done up to that point.
Suited for Success
I was debating between this and Sonic Rainboom, partly because the first time I saw it, I came in about halfway through a stream, so it would be nice to experience it as a whole for the first time. But no, Suited for Success all the way. Because it’s easy to forget that up until that point, Rarity wasn’t actually a good character. So leave it to Suited for Success to come out of nowhere and completely legitimize her character. Indeed, SFS is a microcosm of the show itself; you expect it to be dumb and girly, but it’s surprisingly deep and compelling.
A Canterlot Wedding
For all of its faults, A Canterlot Wedding is nothing short of cinematic. I think every other double episode was trying to duplicate its success, so I’d love to experience that initial impact for the first time again.