Where do cables upgrades have the most impact?

IME, power cords and interconnects have the most affect on sonics.
I like to start an upgrade with interconnects since they influence the timbre and sonic signature the most. They tie the components together. I start with the source.

My power cords are upgraded depending on the strength or weakness of each component. Often the most noticeable upgrade will be the power cord coming from the wall.

I don’t understand it, but in my experience, clearly the biggest impact came from improving the power cord. Interconnects and speaker cables provided nuances, but power cords lifted veils, as they say. Not just replacing the stock cord, but moving up the food chain, in my case from nordost shiva to Vishnu to heimdall II. Sources improved more than my bel canto mono blocks, but all power cord improvements better than any other cable upgrades. Can’t explain it. 

@teo_audio  I realize this goes against the grain of what you posted : )  however, do you have specific advice on how to move along the 'journey' more efficiently?

Answers will vary basses up ones individual components that comprise the system, and cleanliness of AC power.
1) Source to the preamp, (interconnect).
2) Power cables.
3) Speaker Cables.
4) The second set of interconnects if used.