Princess Luna, there're so many reasons why I like her the best.
The simplest is - she has many things in common with me, the dark side, behaving strong in front of the crowd and has a soft heart and i think most of time...stay along.
In her first episode, Luna Eclipsed, her domineering acts just like what I did when I was an angry teenager. About the the dark side, oh god, it was turly a dark time in my life...
And of course, she is always cares about her subjects, I cried inside after watching the S5E13: 'Do Princess Dream of Magic Sheep? ', seeing my waifu suffering from the guilty of the past and she just couldn't let it go...I don't know how to describe that feeling, it's so complicated. But I like her even more after knowing she DOES really care about others, accept the faults her made(sometimes, I can't accept the faults I made and feeling uncomfortable) and become a better pony! All Hail Princess Luna