All 4 of Equestria's princess's together, a truly, lovely job! Here's how they rank with me as favorites, #1.Twilight Sparkle, Princess of Friendship and Harmony Elemental of Magic, #2.Luna, princess of the moon/night, #3. Celestia, princess of the sun/day and #4. Cadance, princess of love! How do you rank them?
Actually Twilight is the princess of magic. its what her cuite mark represents, its her element of harmony, and its (just like the other princess' descriptions fit them) her job to change/control magic... she did become a princess for fixing a broken spell after all.