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

Until recently, I would have said the right speaker with the right room treatment and speaker position. The right components that provide the finest resolution possible.

Then recently I discovered just how much cables can effect imaging.

The size of the image, placement of performers within the image and the amount of space between performers were significantly improved by using great cables. And I mean all cables - interconnects, speaker AND power cables.

Cables allow the entire system to reproduce the smallest details, like the echoes and reverberations within the venue that provide the sense of space within the venue itself and ultimately the reality of live concerts.

Cables will even allow modestly price components to excel and sound amazing

Just my five cents worth - Steve :-)

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.