Because there is clearly a hierarchy.
  1. 5. Pinkie Pie
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    PP is the least coolest pony because, quite frankly, she so damn annoying. From her laugh to her tricks/pranks to her general joyfully-over-the-top personality, she will quickly give you cavities with her sugary sweet antics.
  2. 4. Twilight Sparkle
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    Her element is magic but she's got no personality. The only reasons she's not the lowest rank is because she's a unicorn filly and she's nowhere near as annoying as Pinkie Pie. She later becomes a princess which is kinda cool.
  3. 3. Apple Jack
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    Apple Jack has some fire! She's got a Southern accent and her element is honesty. She's an Earth pony so she can't fly but she's still pretty cool!
  4. 2. Rarity
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    Rarity is a unicorn pony who owns a fashion boutique. How cool is that!?! She also has a great English accent which makes her instantly cool. (As do all British accents.) Her element is generosity.
  5. 1. Rainbow Dash
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    It's no surprise that Rainbow Dash is the best pony. She controls the weather as a Pegasus pony and her element is loyalty...which I value very highly. She has a pet tortoise named Tank and she's just awesome.