Where do cables upgrades have the most impact?

Assuming all existing cables in a system are atleast mid-grade (not junk), which cables/interconnects should be upgrade first and in what order? Where should one start?
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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.
I got a Cardas digital cable for CD transport to tube DAC & it’s made quite an improvement.
I wasn’t sure if it would have much effect.

There is a new level of clarity & accuracy. It does reveal the limitations of some recordings as before there was a veil of ‘bloom’ that had an effect of homogenizing...
I have multiple "High End" cables, and EVERYONE of them have been a disappointment. The LEAST disappointing expensive cables I have are Cardas and since Cardas bought a foundry, it turns out that while they are proprietary, meaning Cardas doesn't use others designs, Cardas is the primary source for an overwhelming number of manufactures. Most companies don't want to do small runs of wire for small manufacturers, Cardas tends to stand alone there. Now, all that said, Dualund makes a wire based upon the old Western Electric wires which they made for AT&T. It absolutely blows away all the boutique brands that I own or have tried. It does need about 8 hours to sound good, and will continue to improve a little bit more over time, but at 8 hours you'll know that you have a winner. You can get SE cables for under $100 on eBay, a character that goes by Grannyring I believe makes them for I believe more, but I make my own. I think Partsconnexion may make them for you as well. Inexpensive Switchcraft RCAs work fine. I used to do minor modifications for people, and still mod for myself and personal friends, though I am running pretty low on Black Gates, my favorite electrolytic capacitors, etc. The point is that I learned about these from a master modifier, Don Sachs. Don makes some of the best sounding tube gear I have ever heard, in fact, he makes the most affordable gear, bang for the buck that I have ever heard. It's what he uses for hook up wire in the audio path. Frankly, even if you pay up for the Grannyring cables, they'll be by far the best sounding cables you have, and by far the best priced cables that can be considered high end. You really can't afford not to try them first, nor can you justify rationally not trying them first. Once you do hear them, remember Don Sachs! He has found some of the least expensive, best sounding components that I have heard. I have used Mundorf Supreme Silver Gold caps, V-Caps Teflon caps, though when I was doing mods the copper foil caps were not out, but Don has found more affordable caps which are awesome, which is partly how he is able to compete with high dollar equipment. For some, his gear may seem unaffordable, but after decades of buying and selling gear, I can tell you that when you consider how much you'll spend over time, and not achieve a system that doesn't impart listening fatigue, his amps and preamps are a true bargain. Anyway, just do yourself a favor and make or buy some Duelund Wire 20awg Copper & Cotton/Oil Hook-up Wire sale price USD $5.77 list price USD $8.25 You Save USD $2.48 (30%)SKU#: DUELUND-81388

For less than $20 including shipping, you can get 2 meters of wire, they also sell Switchcraft connectors probably for $5 +/- a pair, so all in for well under $40 you can own the most neutral cables I have found in 40 years of looking and spending. You could also try to find genuine Western Electric wires, but those who have compared them prefer the Duelund ICs, probably due to purer copper, not to mention that W.E. is being counterfeited as people have found out how good it sounds, so Duelund is by far the way to go!
The long winded answers here.lmai!