Title says it all. It doesn't neccesarily have to be your first episode, just the one that made you like the show. It would be nice if you tell why it helped make you a fan too.
For me, it was Sisterhooves Social. I'm the older one of two siblings, so this episode really spoke to me on a personal level. I really saw a lot of myelf and my younger sister in Rarity and Sweetie Belle, and a bit in Applejack and Apple Bloom too. It was my fourth episode, but none of the other three I watched offered much in terms of emotions like this one did. It was the one that made me want to watch the whole series. Yes, it was that good.
Okay, enough gushing about Sisterhooves Social. Which episode made you a fan?