What is the most important cable in your system

I agree with the deooney. I have found that the most important cable is the one between the Amplifier and Preamplifier.

XLR cables really shouldn't have a sound. 

I have a system that has evolved quite a bit over past 7 years with a variety of amps and speakers, in and out of the system.  My dac, streamer, server and power distribution have all stayed the same.  

And while I’ve noticed subtle changes as I swapped out and upgraded all of my Ethernet, XLR and speaker cables, the biggest difference I have found has been power cables.  Most recently, upgrading from the Audioquest Firebird HC to a Dragon HC on my integrated amp has produced the most profound improvement I’ve ever experienced from a single change.  And that includes both cable and equipment changes.  It’s like a different amp.  

Must be synergy, but I never expected my system to improve in so many ways without any downsides…and I say this with equal conviction today as the day after I first introduced the Dragon HC to my system, and that was about a month ago.

As always, YMMV but I’m still stunned by the improvement.

Power is critical to all that follows.

I upgraded all my cables and I am not sure I heard a difference because the only way I would be able to hear the difference would be to have a toggle switch to compare quickly.