What would you say is THE MOST important factor to good imaging?

AC power for sure, but tops is speaker placement. Makes all the difference. 

Balance control knob

IMO a dedicated/treated audio room is tops. Given this, you have the latitude to place you speakers properly for best imaging as well as your seat.

Next, the speaker itself. Most speakers can image well if given the chance, but some do this much better than others.

Next comes the electronics and accessories.

What would you say is THE MOST important factor to good imaging?

That’s easy. The speakers must be connected in phase.