Where do cables upgrades have the most impact?

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

I switched RCA analog interconnects on my FM tuner and couldn't believe the difference from inexpensive Acoustic Research cables ($15 or so) to some entry level Audioquests ($35 or so). Not night and day, but noticeable. Same when I de-soldered the horrid RCA's on a Project turntable and installed gold plated female RCA jacks and ran some other entry level Audioquests. Not night and day, but noticeable and no hum! I frankly can't hear any difference with digital cable upgrades.