Mexes's kick hasn't got as much surprise element(for the audience) as did Zlatman's... coz in D, when some one chests a ball down you immediately expect him to go for the over-head. At least you wouldn't really be hugely surprised if some one does that after a chest down.
Zlatman's shot was clearly tougher... You know it was not only a volley but it was a shot sending the ball opposite to his run!!
When was the last time you saw a player scoring from a volley, sending the ball against his run all the time by attempting an over-head kick!? That's right, Zlatman's goal is unique! Mexes's, as awesome as it is, isn't!!
Some people are pointing out that Zlatman's kick was into open net, thereby declaring it less credible... But then when a player is attempting an over-head, little does he consider about the keeper's positioning, it's all about the surprise element & testing the keeper's instincts. HE's JUST AIMING INTO THE DARK, going for an audacious attempt! If it ends up in goal, he looks cool; if not, he looks a total fool!!
So IMO it doesn't really matter that Hart wasn't in his position.
Considering these points... Zlatman's goal > Mexes's goal. Not at all downplaying Mexes's finish btw, top drawer stuff that one