What is a bad/average/good/great coach at each level?

So I have been a coach heretic in the past-- they've represented the extra level of tedious management that took enjoyment out of sims for me.

BUT...

After digesting u/tedsternator's (amazing and comprehensive) player development post, I've decided I'm actively handicapping myself by not using coaches. So as I am trying to load my teams up with the best coaches possible, I'm constantly stuck with the 'what is "good" actually?' question.

So I get that in the MLB I should be looking for 'excellent' if not 'outstanding' or 'legendary', but what about Single A, or AA? At what point are the lower dev and mechanics ratings detrimental to the level-specific development needs of the players?