One of the best mons from this gen in my opinion is this prideful mighty duck.
With leek as its item, it lands critical hits almost all the time! It has become pretty funny, actually. 
"Oh darn! this powerful opponent seems really bulky! or "This mon has really buffed its defense"    -----critical hit----- k.o.

First impresion is one of its best moves. It is fast and my have a critical hit on the first round, taking down psychic types.
In the wild area, even if we encountered high level mons, this guy`s critical hit priority move has helped us take many of them down. 

Enjoy!