So I did one for Applejack

 

http://mlpforums.com/topic/13879-12-reasons-why-your-favorite-is-best-pony/?p=1591554

 

and one for Princess Luna

 

http://mlpforums.com/topic/13879-12-reasons-why-your-favorite-is-best-pony/?p=1601956

 

Now it is Twilight Sparkle's turn.

 

1. She has an inquisitive mind and a thirst for knowledge and learning: A healthy mind is one that is eager to learn and Twilight has shown this consistently throughout the series. She can be somewhat closed minded at times such as when she literally judged a book by its cover in Bridle Gossip for example but has improved greatly on this.

 

2. She does not let her status go to her head: As a pony from Canterlot, princess Celestias apprentice and most recently an alicorn princess she has not once despite her privelged background ever become stuck up or looked down on anypony.

 

3. She proves that you can still be cool even if you are a bit nerdy

 

4. Is a well balanced character: As the element of magic which is supposed to embody the other 5 elements of harmony Twilight Sparkle also in some subtle ways embodies some of the positive traits of the other mane 6 while having her own unique character strengths as well as flaws.

 

5. Is very dilligent and hard working: She may not buck apples and may not be training to be a Wonderbolt but she has perfected her skills as a mage through years of hard work and study. It is true that has some prodigious talents but talent means nothing unless you are willing to put in the work to nurture that talent.

 

6. She is so cute when she has a freakout: and I don't just mean the Lesson Zero level freak outs either even the subtle "woah" moment she had in Spike at Your Service when Applejack moved her inkwells around was adorable.

 

7. Tara Strong is one of the most awesome voice actors of all time: seriously she really brings Twilight to life.

 

8. Shows that hard work and diligence is good when tempered with balance: social interaction did not come easily to Twilight at first but as she slowly learned how to do it she realize that it is important to have balance in her life.

 

9. Is very relatable to people who struggle with social interaction: as someone who grew up with Autism I had to like Twilight learn how to interact with others because certain social cues did not come as easily to me as they do with most other people. Twilight may not have Autism but her struggles with social interaction still remind me of mine.

 

10. Has had some excellent growth as a character: she went from the socially awkward yet gifted student of Celestia to now one of the most respected ponies in Equestria. I didn't agree with twilicorn at first and don't agree with its execution so far but am now neutral yet optimistic after seeing Equestria Girls.

 

11. Can be a good voice of reason: Twilight and Applejack compliment each other very nicely as voices of reason and in some cases have kept the other in check.

 

12. Is a good sister/mother figure to Spike: I think that much of the reason Twilight treats Spike as her "#1 assistant" is out of his desire to feel useful and to want to contribute. Twilight indulges Spike in this though it may not be the best thing for him in the long run out of a genuine love for him.

Source: 12 Reasons Why Your Favorite is Best Pony