Which power cord should you buy first

Only one thing matters to me E=IR  

NONE

Salc

First I'd make sure to have high quality power receptacles, something like the Furutech GTX-D (G gold or R rhodium), Oyaide R1, or even the cost effective Acme Audio Labs silver plated 20 AMP cryo and CFC coated at $60

Then as others have suggested; power conditioner, streamer, DAC, preamp, and amp. The signal components don't need as heavy a cable, so usually a little less expensive; the preamp and amp I prefer a heavy gauge, something like 11AWG or less.

My experience is that the most noticeable changes to a device when a alternate Power Cable is added, is on SS Devices. In my case this is a CDT.

When adding the same Power Cable to the DAC, change to the end sound is detectable but not to the measure of the CDT.

In my case the Power Cable selected also has produced a change to the end sound that is perceived as a betterment on both devices.  

It pays to experiment, but with my specific gear, power cable was better on the source first.