Ok the fourth panel Dash is really cute.  
And Big Mac expression really sells it.

@*Rainbow Dash*  
Fluttershy is at least as different in Rainbow in lifestyle. Personality and interest wise AJ is far closer.
 
Rainbows relation with Fluttershy comes off more as family.

@*Rainbow Dash*  
Yes we can agree on that

@Background Pony #D09A  

 
thats what i was getting at overall is that theres no way Dash is picking one over the others in general. and the same thing could also be said about Dash and Twilight and their fondness and enjoyment they get out of doing ‘‘nerdy’’ things together like reading and Daring Doo and what not.

@Background Pony #D09A  
plus they love constant challenges

@*Rainbow Dash*  
Both aj and rd are very passionate about sports in general, games, going outside, both are hardworking (yes rd can actually be hardworking otherwise she wouldn’t be a wonderbolt) and competition, not to mention the fact that rd often comes to help aj (like that one time she was destroying the barn), and yes even the bestest of friends fight and argue sometimes. Just because you know someone longer doesn’t mean they have to be your best friend, i think dash has more fun with aj than with fluttershy and her boring tea parties.  
But again i really doubt dash would pick a friend over an other.

@Background Pony #D09A  
except they dont… other than they are both stubborn and prideful and like to compete, they live completely different lifestyles that share almost nothing in common. and with Fluttershy Dash has known her twice as long and in the show they’ve been shown to push each other to be better ponies.

@*Rainbow Dash*
 
Eh Applejack and Rainbow have a lot in common honestly, no I’m really not surprised she calls her ‘best friend’ *wink wink  
Fluttershy is more of a sister to her, but they don’t share many common interests

That RD in that 4th panel is too cute! :3

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.

@Incursor  
In the present yes. In the future on the other hand…

So Applejack’s her best friend out of the Mane Six?