"Ilia is definitely the hardest hitter"

[Söze Aldo]

There are guys like Emmet and JG who hit hard in the sense that they are throwing 100% winging punches. Justin has legimiately thrown himself to the ground in multiple fights swinging. Josh winds up and swings for the fence.

Both of those guys 'hit hard' but they are winding up their shots and ripping to their maximal effort.

Illia is clean. He's hitting guys fast, accurate and without exaggerated effort. He's heavy handed in a way that is fundamentally different than those other guys.

He's precise and almost always lands on the chin, which is right where you want to hit someone to turn off their lights.

The other guys you referenced, since they are often winging their shots they will often hit the guy on the side of head, which is one of the hardest parts of the skull and while it will make someone discombobulated, it usually won't fully put them out like a clean chin strike.