Jun is allready kind of a mess a bit so without him that would be the worst
I can see it allready ...
WOW: He would be like "key, I'm the leader ... idk what to do" *visible confusion* than smiles and acts like if nothing happend
Byeongkwan: "Why do you even ask me to be the new leader? I can't do it... why me?" if people than would say it is maybe cause he is such a good dancer he could panic and scream at you probably "I'm allready working 2 jobs so why should I also lead my group?"
Chan: *laughs than panics and runs A.C.E against a wall*
Donghun: In the worst case he could diss everyone 24/7, than say "Who cares" and end it by a big sight of "Jun wasn't even good, so why was it me to replace him?"
So I would say: nobody