Sunset Shimmer; her story is one of forgiveness, growth, and acceptance. Sunset was an enemy flat out in the first Equestria Girls, but now has become a full on main character in the series; her stories are the most compelling, in my opinion. Sunset is broken, and constantly seeking acceptance knowing what she did was wrong, and struggling to move on with herself after making a huge mistake, something we all probably have done, and sometimes struggle with. Letting things go, and moving out of that darkness that consumes her really makes Sunset seem relatable; and in friendship games, when she openly yells at Sci-Twi, it is obvious that she still has a bit of her mean girl brashness to her.
She is a pretty awesome character.