S1 E2 - "Friendship Is Magic, part 2"
S1 E13 - "Fall Weather Friends"
S2 E5 - "Sisterhooves Social"
S2 E15 - "The Super Speedy Cider Squeezy 6000"
S3 E6 - "Sleepless in Ponyville"
S3 E13 - "Magical Mystery Cure"
I personally love "Magical Mystery Cure" because in the episode, the destinies and cutie marks of Twilight Sparkle's friends get switched around, forcing Twilight to create a magic spell of her own in order to correct it!!