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

Agree with everything mentioned above (even roberjerman). In addition I would add control of micro-vibrations via feet, racks, weight (on top of components), etc. It all makes a difference. 

Obviously everything matters but number one on my list would be speaker placement.

Speaker placement, but you have to have a room that allows a good placement. While I've been able to achieve excellent horizontal image placement and a very wide sound stage in a small room, getting great depth has, for me, required that speakers be positioned more than a couple feet from the front wall. Six or seven feet into the room works very well. I've also found that dipole speakers (planars or open baffle) tend to provide more image depth than monopoles. 

The quality of your A/C and the lowest noise-floor electronics you can afford.

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