The most important factor in soundstage imaging

Before we get all technical, and deal with speaker design, placement and room treatment, the beginning, for me, is the recording. I have many different copies of the "same" record; some sound flat and lifeless, others have dimension. That's not the sole factor, of course, but for me, it's where it begins. Even with a good system, well set up in a room, a lackluster recording, mastering, etc. will sound very disappointing, particularly when compared to a better copy from a better source, better mastered, etc. All of the other elements in and relating to the system play their part, not necessarily in the order of the chain. Rolling a tube can make a difference in presentation. A phono cartridge change can make a profound difference (as can its set up). 
I don't have a holistic view of the entire picture to present in some logically ordered fashion of priority, but let's not forget the role that the source material plays in this process. 

@n80- I think part of it is the context of the board, which is gear centric. No disrespect intended, since many of the contributors here are well established,knowledgeable, etc.
I was stuck in "audiophile land" for decades, listening to what I now consider to be pablum- sonic wonders with the musical substance of cotton candy. Jumping into record collector world, it’s the opposite- pressings treasured for their rarity, not necessarily their sonics (or even the music).
To me, it’s fun to straddle both worlds and between them, find music i enjoy that sounds good. I geeked enough to build a good system or two over the years, and hung out with the record collectors enough to learn about the deadwax, pressing plants, mastering engineers and labels. But ultimately, it comes down to how it sounds. And that’s where I think the audiophiles and record people sometimes overlap. (The music itself- the performances, the compositions, the playing, is yet another whole layer of stuff- perhaps the most important).  
I’m in a mood, so please don’t read this as snark. It’s meant more as self-criticism, if anything. I can always learn something about both worlds--that’s why I hang out here, on the gear side (and occasionally, somebody mentions an interesting band or album). But, this is a gear centric place, record collecting is about the rarity or obscurity and there’s a whole other world that is about the music itself. It’s a happy thing when all those different strands combine to make magic.