Which power cord should you buy first

Power amp

…….why don’t you take some time of your own and research your question that has been covered here on Audiogon …..ohhhj at least 1,000 times . 

Given the list you have go from left to right. If you are listening to the turntable you could unplug the DAC and streamer, so those cords wouldn't matter, and vice versa, but: 

The most important power cable is the one going to the power conditioner. Your best power cord should be on the power conditioner. After just upgrading the cord on mine I agree with this philosophy. 

According to Shunyata the system as a whole will not be good until all power cords in the system are upgraded (if one uses theirs) as each cord provides shielding but also their own filter and stops the noise from the component going back into the rest of the system. So if one had $1000 to spend their advice is the system as a whole would improve more to put $250 cords on each of your 4 components instead of $1000 on one or $500 on two of them.

If there is a bottleneck in your system you won't get the full benefit of a major change on one component.  Include the dedicated line(s) and outlet and ethernet cabling and filtering in this chain as well. 

I could have as easily given the answer @garebear has, depends on my mood on any given day. 

But agreed, right at the top of this page, see the "search discussions" bar? search first, then search other forums- that is what I do instead of starting new discussions.