Where do cables upgrades have the most impact?

Well...after reading everyone’s responses what I surmise is that opinions differ because everyone’s system is different and cable upgrades impact systems differently. So...the old audio adage of you have to try it and see how it sounds to you in your system in your environment holds true with cables as with almost everything else in audio.

yep

Most online cable dealers offer a trial period included with a sale.
So, the next step is to decide on which brand to try.

I would advise introducing one cable at a time when auditioning.

Speaking of brands and models, you will want to try what you never or almost never see used for sale yet what sells new in significant quantity. I am talking real cables and cords not $200 entry level junk.
How would you know ? Not sure, find a way.

To answer your question, mostly on your wallet.