Fun Fact: Season 5 was when I started watching FIM, although it was already close to the Season 5 finale by the time I started.

5.Party Pooped — This episode introduces us to the Yaks and Pinkie Pie's Party cave, the latter of which explains how Pinkie is so good at preparing parties. Also, did you know Twilight is afraid of quesadillas?
4.The One Where Pinkie Pie Knows — Not so easy to keep an important secret when you're the one doing it, is it Pinkie?
3.Castle Sweet Castle — It was nice to see Twilight's friends pitch in the make her new castle feel as much as a home as the Golden Oaks Library did.
2.Rarity Investigates — Another mystery episode! This one shows that Film Noir and Rarity go pretty well together. This episode may as well be called, "Rarity's Feminine Charms."
1.Slice of Life — It's refreshing to see an epiosode where NONE of the Mane 6 are the focus.

Disclaimer: These "top 5" images are my opinion, so you're free to agree or disagree with them.