Everypony knows how competitive can this particular duo get - Rainbow Dash and Applejack.
So just a few weeks after getting the Elements of Harmony, it's natural for Rainbow Dash to start thinking about who got the best Element.
What she uncovers will change a lot.

I've started this story a long time ago, hence it takes place early in the series (duh) - that means unicorn Twilight and the Elements of Harmony!