#1 Big E
The New Day has been a unit for quite a while, so you could make the argument that it's probably the correct time to split them up.
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As for who will be the one to turn, well, that's an easy one, as a Big E heel turn would probably have the biggest impact. Sure, this move might leave Xavier Woods in limbo, but it'll also give him the opportunity to get over on his own, which he is most certainly capable of.
Of course, at this point, it would be ludicrous to turn Kofi Kingston, as he has the chance to be one of the most popular WWE Champions that the company has had in quite some time. Also, Big E would get a ton of heat if he does end up turning against his longtime partner out of jealousy.
A Big E heel turn is something that no one would see coming, especially coming off of Kofi Kingston's big feel good moment at WrestleMania 35.