Twilight Sparkle

Twilight Sparkle is the main protagonist of My Little Pony Friendship is Magic. She is a female unicorn pony who transforms into an Alicorn and becomes a princess in Magical Mystery Cure. She is also the daughter of Twilight Velvet and Night Light, the younger sister of Shining Armor, sister-in-law to Princess Cadance, and paternal aunt to Flurry Heart. She is voiced by Tara Strong.

Why She's the Princess of Friendship

 * 1) Massive character development. She starts out as an anti-social unicorn who only wanted to study to an outgoing princess of friendship who makes use of teamwork.
 * 2) She is a very good fighter, as she was able to defend herself against multiple opponents, such as the Changelings and Lord Tirek (with the other princesses' magic)
 * 3) Cute and original character design.
 * 4) She takes good care of her dragon Spike.
 * 5) Shown to be forgiving, like when she has helped to reform antagonists, such as Moondancer, Starlight Glimmer and Sunset Shimmer.
 * 6) She was introduced to the Heroes Hall of Fame.
 * 7) She tries her best to be rational and does not approve of accusing people.
 * 8) She is a skilled flyer.
 * 9) She is a great teacher of magic and friendship. She even now owns a school.
 * 10) She was the only one able to work her own way out of Discord's discording when everypony in Mane 6 turned into the opposite versions of themselves.
 * 11) She managed to change the way girls' shows look.
 * 12) She is very intelligent and knows how to work her way out of any situation.
 * 13) She is one of Tara Strong's most iconic voice roles, right after Bubbles from The Powerpuff Girls and Raven from the Teen Titans franchise.
 * 14) she's a fucking sexy for a unicorn, gimme that horse pussy
 * 1) she's a fucking sexy for a unicorn, gimme that horse pussy

Bad Qualities

 * 1) Sometimes she is not very rational, such as when she intends to act friendly towards Princess Skystar only as a distraction so she can steal the transformation pearl.
 * 2) She was once an annoying and somewhat unlikable anti-social unicorn. When Celestia said she should make some friends, she was actually Sceptial towards her. Her own teacher.
 * 3) Following up BQ 2, she is skeptical towards multiple beliefs, even when they are true.
 * 4) For some reason, she has a fear of quesadillas and ladybugs, all because, as she quoted, "They're just so... cheesy."
 * 5) She can easily lose her cool under stress. In Lesson Zero, when she can't find a friendship problem to solve, she goes crazy and even creates her own friendship problem. And the way she acts in this episode is very disturbing.
 * 6) She has been flanderized in Pony Life and became a psychotic nerd.