Where do cables upgrades have the most impact?

rather than buying 4 new power cords at say, $400 each, buy a power conditioner/surge protector that delivers stable voltage and current for around $1000. I'm in Australia and purchased a locally made unit, the Thor PS10 which comes with a first class power board as well that has both coax and CAT cable protection. I went down that road, thinking more of equipment protection due to a previous issue, but I am pleasantly surprised by the sonic improvement to my system in general.

I put in some interconnects that's made my system a joy to listen to.  I can't tell you which wire will do that to your system if any but good luck.

In my evolving system for the last 10 years I have found the power cord to have easily the greatest impact (this after a conditioner [Shunyata Hydra,Audience RPT6, Audiodharma Powerwing, etc.] or after my current regenerator [PS Audio P10]), followed by interconnects, and lastly speaker cables. 

Interestingly, I am an Elrod cable fan, and David Elrod flatly states that he finds the speaker cables to have the greatest impact (and he enjoys a lot of feedback from a broad range of customers).  I'm guessing it all depends upon systems and tastes?

It also depends on wall current.

Get some really good interconnects and then a decent power cord, or two. Speaker cables will fall into place much easier and for less cost.

All the best,
Nonoise