Well, if I were to take a pony based off of their given reasons, I honestly wouldn't take Applejack. It isn't that I don't appreciate that she wants to help the Apple family; I just wouldn't want to take a pony to the Gala only to not get to spend any time with them actually IN the Gala because they're outside selling Apples. Besides, Applejack can sell her apples just as well back in Ponyville.
I would probably take Pinkie Pie based on their given reasons, simply for the fact that she wants to party, and that would equate to a good time with her, rather than A) Sitting around an apple stand, B ) Listening to Twilight talk to Celestia, C) Getting dumped by Rarity for a prince, D) Being ignored by Dashie in favor of the Wonderbolts or E) Walking around in the garden and listening to Fluttershy complain that the animals don't love her.
However, if the pony's given reasons were out of the picture, I would take Fluttershy. She is the cutest pony, and also I just love her personality.