In terms of alignment with their Element, Fluttershy has always stuck out hard as being the farthest way from what would be necessary to reactivate it. And I say that with the full acknowledgement that this entire thing was Captain Dash's fault, because even with her it was more of a loyalty towards the wrong thing scenario than it was an outright lack of loyalty, much like it was in S2E1.
Assuming the Happy/Sad switch is what needs to be done to snap Pinkie back to relative normalcy, I think she would be the easiest to be able to represent her element. And considering the reason Pinkie is the way she is, I think that all that needs to be done is give her a little hope.
Rarity has already basically gone through the attitude adjustment that is required of her, so she's probably golden even now.
Applejack knows what she needs to do, and probably knows how she needs to do it, but she is so far refusing to do so because... well, because she's Applejack.
Fluttershy, however, not only will still need to overcome the reasoning that she used to justify her (for lack of a better word) corruption of what she is supposed to stand for, but she will also need to overcome the atrocities that she will feel she committed while she was commuting her behavior. And the full weight of what she has been doing is going to hit her hard once she realizes that her reasoning has been nothing more than a bunch of rationalizations she put in place to defend herself, and if someone (probably Dash, because of how Dash reacted when she realized what Fluttershy was doing) doesn't comfort her when that happens she is going to BSoD something awful in response.