Where do cables upgrades have the most impact?

Is it really the cable industry that needs our excess money the most?

The ’industry’ exists because someone who BUYS cables wanted something better. One ’better’ cable at a time... It grew. If no audiophile bought them, the industry would not exist. One actual plus for consumers is cables generally do not wear out. I still have cables that I made in the 1980’s. So aside from wanting better... no need to ever buy more. Cable company advertising is only to lure you away from SOME OTHER CABLE COMPANY. No one walking down the street accidentally saw a cable company advert and decided Gee I must NEED those bad, and rushed to buy some.
I am probably like a lot of audiophiles in having a pile of cables I do not use anymore. All told between IC, PC, speaker plus video or digital I must own thirty cables not in use, sitting in the closet. (anyone need a Monster S-VHS cable??? I only keep one because.. never know I might suddenly want to use it.. for????)   
As for "the most".. Priorities. Sometimes you might be best off spending your money on some other audio doodad. But sooner or later you might, maybe, decide to spend that $3000 sitting around on a cable? Who knows.. I did. (and glad I did)

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?