What is most important part of a system?

The most important part of any audio system is the person setting it up. The individual who puts up the rig controls everything from the larger parameters of performance, such as cost, to the minutia, such as listening position and position of speakers.

I have for years stated, "The single greatest impediment to building a high end system is the audiophile." Why say that? Because most audiophiles inhibit system development through mistakes they make. They do not wish to spend enough money, or they do not care enough to find highly synergistic gear, or they ignore aftermarket power cords, or a better digital source, or USB cable, etc. Most, perhaps 75%, of audio systems are not that good because the person putting it together doesn't care all that much, or is chintzy, or depends on someone else's advice versus actually trying several things to improve it.

It should be obvious that ALL parts are critical, that any one which is poor can submerge the performance. It's self-defeating to discuss which one component is most important. That is like asking, "Which parts of this system can I skimp on and still expect great results?" It's a loser's game. :)

Don't expect cheapskates, who are always looking for a way to skimp on it at some point(s), to build a superior system. :(