Where to put one's best power cords??

...first of all: EVERYTHING ( Electronics, interconnects and speaker  cables, powercords) NEEDS SOLID BREAK-IN TIME.
I do 300+ continuous hours before any serious listening.
Find a dealer or friend who will let you experiment with different combos; vary your music greatly, and vary volumes you listen at.
BE FORWARNED: THIS IS TIME CONSUMING PROCESS AND SNAP DECISIONS ARE USUALLY SHORT-LIVED.
My present choice of power cord into my JOB INT AMP is a PS AUDIO SC STATEMENT...

Some or most power cords need in fact more than 300 hours to reach their full potential, but yeah 300 hours should get you most of the way.
Shunyata power cords are widely considered among the best overall. I thought about getting one but then the Dominus came up. I could use couple of better cords, though, on the phono and PS Audio regenerator. But this would be expensive and not so easy, Custom Power Cord Top Guns that I have are quite good, so it would be at least $500-$600 each used, thinking either Purist or Shunyata.

Jeff - the 4th response from Elizabeth was the right one. Experiment and confirm where the aftermarket power cord does the most good. While there are general rules that are true, there's no substitute for experimenting and confirming where it's used to get the best improvement.

Go to Steve Reeve’s website (image99.net), click audio alchemy, and build his DIY power cables.  I built four cables for under $450 (including silver plated connectors and Furutech signal wire) and connected everything.  Tremendous improvement.

All good wishes

1. Wherever it sounds best.
2. Wherever it looks best.
3. Wherever it fits.