Applejack was introduced in the first episode where her massive family magically showed up to greet Twilight and Spike. They filled Twilight with a variety of apple foods, presumably with the intention of cooking and eating her later.
She then had her first episode of focus in Applebuck Season, where she decided to troll all of her friends by messing up the job she offered to do for them. This trolling led to half of Fluttershy's bunnies running away, and led to a mass food poisoning in Ponyville, due to Applejack poisoning Pinkie Pie's cupcakes. It can therefore be deduced that Applejack is about as dastardly as the Joker and seeks to poison the entirety of Ponyville. She was taken to a court off-scene, tried, found guilty, and as punishment she was not allowed to have any more episodes to herself until Season 2 episode 14.
The next episode saw Applejack fuck up a rodeo she thought she'd win, so she got sad and decided to ditch all of her friends. When her friends came to help her, she pushed them away, obviously meaning that Applejack is a stuck up pony, much like Rainbow Dash, and therefore everyone should hate her.
Wait, what am I talking about! Applejack actually died during the episode, because for a kids show, death is the most obvious reason someone goes missing. So R.I.P. Applejack, you will be missed. The episode was actually about Derpy Hooves.
She is supposed to represent the element of honesty, but she refuses to tell her friends about all of her problems, so that makes her a dirty liar and gives us even more reason to hate her.
Reliable sources state that Applejack is a Mary Sue, that she has no flaws. This is another reason to hate Applejack more than Stalin himself.
Bronies, being the respectable community they are, have unanimously agreed that Applejack is worst pony, because ponies who don't talk much like Fluttershy, or ponies who like to show off all the time, like Rainbow Dash, obviously have better personalities. It's not like Applejack is always around to help her friends or anything, or making sure they feel better if they're upset. Wow, just such a mean character.