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

Time and phase accuracy which includes pistonic motion drivers :-)

Contrary to George’s opinion, horns can provide excellent imaging if somewhat reduced SS depth. Just another pointless dig at horns he feels he must insert. Oz

The more directional the better, that’s why esl’s, ribbons, planer ect are so good at imaging, width, depth and placement. But are usually one to two person hot spot seating, horns just have a diffused "wall of sound." 
Iv’e owned LaScala’s, Heresy’s and Forte, they don’t come close to esl’s, ribbons, planer ect.

Cheers George

Well that's that then... Let it be known horns do not image (looking for gavel to hit table)

#1 would be getting rid of all reflections in the room . Obviously not all . Third wall reflections and on will not have much energy anyways . Try it out . Buy some Owens Corning or Johnsmanville 2'x4' rigid insulation as a test in your room . Under $100 for a 9 sheet pack where I live. Buy 9,18,27 sheets . Do your testing . Keep the stuff or buy professionally . I have moved a few times recently and listed hundreds of sheets on craigslist and sold it everytime I listed it at 3/4 of its retail . 

Program material/mastering. If it’s not there, no speaker placement, no cables, no room treatments, no electronics will create it or correct it or enhance it.