After having worked with Howard and BL I would have to recommend Chip Gregg at NKU especially after working with Lizzy Lemire(ND Hitting Coach).
Chip & Lemire are extremely similar in their emphasis of attack mode and positioning.
We had great handpath and tracking and various other mechanics and skills those top three preach, but until we worked with Chip and Lizzy(ND) on ATTACK mode and how to really explode into the ball and continue through with back side, DD then went from .400 singles hitter to a .450 hitter and slugging .800+
I know Howard, Bustos, and BL all know and teach that stuff, but Chip and Lizzy make that their emphasis and a kid really starts to understand how to store and release energy.
High recommend all of them (Minus 1) but would seek out Chip Gregg or attend an ND hitting clinic to really unlock the power within your DDs.
IMHO