Okay, Applejack has been my favorite pony since the start so get ready fillies and gentlecolts cuz I am going into fanboy mode.
1. Applejack is the most true to herself: I like people or in this case ponies who are real, genuine and true to themselves. Too many people put up these false fronts to hide who they really are so they won't be judged but Applejack dosen't care about any of that.
2. She has strong morals and is the best role model both for the young children and even the adults watching: Applejack is not perfect as her pride and stubbornness can sometimes make her fail to see the big picture but she always strives to keep her word, do the right thing. She can be a bit hard on herself when she falls short but her strong code of honor is very refreshing in a character.
3. She is very hard working and diligent: Applejack has a good can do attitude and a work ethic that is quite admirable even if she does overdo it sometimes.
4. Family is important to her: She has always been there for her sister, her brother and her grandmother.
5. She is the most inspirational character in this show: She has had to grow up way too fast after her parents died when she was just a filly by helping her older brother Big Macintosh run the farm and raise their younger sister Applebloom. She has survive a situation which would have destroyed many others and grown into a strong, honest and faithful young mare.
6. She is confident yet humble: Applejack is perhaps the most confident of the mane 6 but dosen't suffer from the arrogance which Rainbow Dash stuggles with though has improved greatly on.
7. Is a good example of an average character done right: Applejack dosen't have as many quirks or flaws as the other mane 6 but if you are willing to look deep enough you will find that she is a character that seems simple yet has a very compelling story to tell about coping with loss, dealing with the pressures of running a farm and a few other things such as her stubbornness which make her a more interesting character than she gets credit for.
8. Applejack is very friendly and welcoming: Applejack gave Twilight the warmest welcome to Ponyville of any of the mane 6 by treating her like she was practically part of the family and is always willing to lend her friends a helping hoof.
9. Applejack represents her element of honesty very well: She is honest often to a fault even when it seemed like the Flim Flam Brothers had won she said "a deal's a deal".
10. Applejack is no cheater: the only time she ever fell back on this was in Fall Weather Friends and it was only because Rainbow Dash was cheating.
11. Is mature yet knows when to let loose and have a good time: Applejack works hard as even the various people who think she is the "boring one" will admit but even she knows when to put that away and party every now and then.
12. Applejack is one of the most well balanced characters in the show: sure she has fewer flaws than the others but is written in a way to where she dosen't need as much to still be an interesting character yet still has realistic and well written flaws as well as strengths.