I really like the fanfiction 'Mirajane: The Hidden Power'. In this story, Mirajane discovers a hidden power within herself that she didn't know she had. It shows her growth as a character and how she uses this new power to protect her friends in Fairy Tail. 'Mirajane's Friendship Bonds' is also a good one. It focuses on her relationships with other members of the guild and how those bonds are strengthened through various adventures.
A fanfiction that stands out is 'The Mirajane Enigma'. This one plays on the mystery aspect of Mirajane's character. It keeps you guessing as the author slowly reveals more about her. There's also 'Mirajane's New Beginning' where she decides to start fresh after some difficult times in the guild. It's an inspiring story of self - renewal.
One great 'Fairy Tail Mirajane fanfiction' is 'Mirajane's Redemption Arc'. It delves deep into her past traumas and how she overcomes them. Another is 'Mirajane and the Lost Magic', which explores her discovery of a new and powerful magic. The fanfiction 'Mirajane's Love Story' is also quite popular as it weaves a romantic tale around her character.