Princess Luna has always been my fav Princess in MLP.
(not including Twilight, she's my fav of the mane 6)

Ive always found her so pretty and personally, i think she's the most developed character.
Not including characters like Twilight or Discord. because most of their story happened ON screen.

For Luna, her backstory happened mostly in flashbacks or tales of legend. Hardly ever played out in present day.
But yet the brony fandom has more knowledge of her than a lot of the other characters, even in later seasons with small fun facts.
Like how Luna prefers to relax and unwind while on holiday, unlike Celestia who wants to try all kinds of thrills and adrenaline rushes.

Plus, the small detail to her of the edges of her mane being rather transparent.
Small details like that really make a character stick out in my mind, and i love Luna for that small detail.