*Rainbow Dash* 
I'm sorry but that line of dialog is dumb..... At this point in Dash's life if she were to have to pick ''one'' best friend (especially within the main six) it would be Fluttershy. As they've know each other since fillies and Dash spent a good portion of that time defending her (hence sonic rainboom). Heck we even had an episode that essentially showed that Dash is treated as a member of fluttershy's family!
although having said that, it's common to say ''my best friend'' in regards to multiple friends when talking about said friend.
although having said that, it's common to say ''my best friend'' in regards to multiple friends when talking about said friend.