Which power cord should you buy first

Your DAC.

Fun Guessing- Power conditioner, Dac, Pre, unless your amp Power cord is too small for your amp, then amps 1st.

Actual -Once you got the one, move it around and see for yourself.

@mclinnguy -

                                                          +1

The most important power cable is the one going to the power conditioner. Your best power cord should be on the power conditioner.

    After all: that’s the source for the rest of your system’s power, unless your main power amp(s)* is/are fed directly from the wall.     In which case: both/all.

             *Mine are and have their own Synergistic A/C Master Couplers.

                                                  Happy listening!

Amplifier. Likely to make a very big difference. Typically twice the difference that would come from other components. 

I highly recommend trying Audioqurst Hurricane if you want transparent sound (that is, exactly what your components sound like) and Cardas Clear Beyond is you think your system might sound a tad better if it had a slightly warmer more bloomed midrange. For Cardas the lower the tier, the warmer the sound. 

Echoing what @mclinnguy said, If you have a power conditioner, I would get a power cable for that. My thinking is that results in cleaner power to all downstream components resulting in less effect of placing a power cable on those other components.

If there is no power conditioner in the picture, I think mainstream advice might be to put the power cable on the amplifier. But, I might suggest a power cable for a DAC. I have found a surprisingly large beneficial effect in clarity and resolution with the right power cable on my Denafrips Pontus II DAC.

However, I will say that especially on my DAC, finding the right power cable was very important. I tested a whole bunch of power cables on my DAC. 6 sounded worse than a basic $15 Tripp-Lite Heavy Duty power cable from Home Depot, 1 sounded about the same in overall sound quality (though it still sounded different), and 3 sounded better than the Tripp-Lite cable.

Too much noise filtering technology (or the wrong filtering technology) in a cable seemed to result in a dull and muted sound. A generic $15 power cable actually sounded better than a whole number of cables costing between $500-700. A $95 power cable ended up sounding best.