Where do cables upgrades have the most impact?

Really depends on the whole system and you begin with the weakest links

If you split them up in digital, analog, power and speaker cables them I would start with the power section. This can trigger a debate with no right or wrong and only standpoints. 

Try it yourself. Feed at least your amp and sources with some decent powercords. I did not need to spend more than 350$ on a single power cable or block.  In general a factor 10 on the source is for me a non scientific rule. Common sense should do the math. Probably cheaper cables where out there that gave me the same results but I can’t try them all. 

In my opinion and based on my own test cases with cables diminishing returns will rise early. 

A test protocol for power-cords  that I use. 

Stick with two or three peaces of music. Listen for bloating , heavy bass  , cool and warm tones (timbre). Do some background reading on putting the devices in the correct fase.  I can’t detect in correct fase as some others do but it is easy to do. Peace of mind will also help. 

Try shielded and non shielded cables. Non shielded gave me less bloating or heavy bass and more alive music. There is no conclusion. 

Choose every time the sound you like the most.  Work your way up the chain and repeat the proces. You have to match and balance out the system. Work with what you have and try to get the most out of that fist. 

Try pro cables to. In most cases cheaper than consumer market cables and the the results can surprise you. 

 I have a mix of factory made (pro and consumer) and DIY cables. The main reasons for that is that I can test and swap cables without a big budget. You don’t need over expensive cables to get good sound. When I get bored listening to the same sound and want something else I have some cable options in my sleeve. 

Most of the time a come back to my reference point. I again know the contrast and enjoy what I have. 

I do reference with friends to check how my DIY cables sound. There is nothing telling me that DIY in the analogue section is worse than factory cables. Digital I leave to the pros. Meicord UTP is I non sponsored tip. I run it in all of my digital chain. For my digitaal section they are the end of the line UTP Cables. 

No Guaranty that my experience will be yours. 

Speaker cables wuld be my recommendation. Make sure that size and length match the amplifiers electrical performance. When this is done it ie easier to listen between the different interconnect cables. Best Mik

The shorter the better? When talking of cables, this statement is often presented as an axiom, although after several experiments it is not even a theorem for me. Using the example of intercon cables (the same manufacturer and the same model), I can say that in most cases a cable 1 or 0.8 meters long sounded better than half a meter long. I suspect that the matter is in some "wave" nuances, but I would be happy to know the expert's opinion.

Curious if those saying the power cable had the most impact are plugging into a power conditioner or a wall outlet?

For me personally I do not filter my power. I never heard a serial filter or / power conditioner to my liking but I have not heard them all.  What I have heard sounded to cleaned up for me. Borring is what came to mind.    

I do have a separate HiFi power group. Does this lead to a better experience ? I don't know. Theoretical it could. 

A power strip in a wall-outlet when you have to connect more than one or two devices is a logical option. 

My power section is build around a good 8 piece power strip that goes in the separate group wall outlet.  From there I have plugged in my devices in order Power AMP and the rest. When I use a non seperate wall outlet I think I cannot pick it out in a AB or blind test.   

In a powerstrip digital devices are adviced to be last in line. I am not able to hear differences in other combinations. I just followed this advice because it cost me nothing and the plausible explanation came from an experienced person.    

I have power cables that are 6 feet minumim lenght. Read many sience, theories and opinions about the minimal power cable lenght.  If the theorie and science is wrong than we waist a lot of material. If it is correct then I have the right cable lenght and created piece of mind in this section.  

Still changing the power-cord on an Pre-Amp / AMP or Digital device like a Dac is a good place to start. 

Start with the basics. Dont over do it and go from there.