I'm just going to cut to the chase: We can immediately cross off Balloon Boy, JJ, The Puppet, and Withered Golden Freddy. For the first 3, they don't seem to have ANY animatronic mouth. As for Withered Golden Freddy, he doesn't have any endoskeleton, he's just an animatronic suit slumped over. Mangle's jumpscare shows her biting the players forehead, and Ralph says that the '87 Victim survived which is simply unlikely given the size of mangle's mouth. And besides, its more likely a withered did the bite since the classic animatronics in FNAF 1 [which is repaired withereds as seen from the night 5 newspaper] cant walk as a consequence of the bite of 87. Due to this, we can eliminate Toy Freddy, Toy Bonnie, and Toy Chica.

We can IMMEDIATELY eliminate Withered Bonnie because of the lack of a top jaw, unless he just...awkwardly inserted it into Jeremy's frontal lobe. Withered Foxy's jumpscare is a major point other theorists try to use, however from the angle Foxy's mouth is positioned to it seems like the bite is going for the eyes instead of the frontal lobe, they also try to use the week before as evidence as well but lets be honest that's overthinking a small line. Withered Chica fucking has blood on her Help Wanted endoskeleton, that has to be a dead giveaway!....right?.....Well, we don't even know if she can close her mouth.

That only leaves one suspect left from process of elimination:
Withered Freddy.