Rarity's best friend is Fluttershy.
They have a close and supportive friendship, often seen helping each other through various challenges and celebrating each other's successes. Their bond is built on mutual respect and understanding, with Rarity appreciating Fluttershy's kindness and gentleness, and Fluttershy admiring Rarity's talent and ambition.
While Rarity may have a close relationship with all the other ponies in My Little Pony: Friendship is Magic, her friendship with Fluttershy stands out as particularly strong and meaningful.