Which Merc do you find to be the most useful/beneficial?
I have a goal of maxing out their …uhh Rapport. Then onces that’s done I’ll side line them and move on to the next. 
As far as hiring is concerned, Aldkin has the detrimental curse debuff and Subo fills mini-map with dots. Both can be annoying.
Varyana has a guaranteed Overpower proc buff and no obvious downside.
I have only Spiritborn in season atm so I just hire Reheir for the armor buff, but I wish developer team would remove his grunts when he gets hurt in combat. He often says “Impressive. Unfortunately.” What is that supposed to mean?
Probably because it hurt.
I guess that makes sense. I thought he says that to the player after killing a bunch of monsters.