Yet another HiE (Human in Equestria) fic, Through the Well of Pirene, by Ether Echoes, stands out, it's quality writing and engaging characters help it to shine through the bajillions of other HiE fics out there.
The story focuses around a teenage girl, Daphne, and her younger sister, Amelia. After having a falling out, Amelia ends up being led into Equestria and turned into a pony by goblins, while Daphne desperately tries to find and save her (she also got ponified by a magical wand that will remind you of the Wabbajack). There's a lot more to this story, including horses, guns, quirky relationships, a long-lost friend, and more, but I won't spoil it for you.
In summary, even if you avoid Human in Equestria stories as if they were so much toxic waste, you should give Through the Well of Pirene a try; you'll be glad you did.
Pruzah sul, dii fahdon.