Where to put one's best power cords??

, Where should one put their finest power cords? Mono amps, preamp, DAC or transport?  Where will the best cords have the most impact?  Thanks, Jeff
Per my conversation with Audioquest, the best power cable should be on the source.
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"That would be the DAC, as opposed to the transport"

Not necessarily -- in a full DCS stack the best place for the first power cord upgrade is on the clock ...

The obvious answer is where the influence of greater power stability will be most impactful but that also depends on how the power supplies themselves are designed.

Unfortunately you probably have to try but as a rule closer to source (i.e. in the original question that would be the transport actually) as any problems created there cannot be corrected downstream
John Rutan at Audioconnection recommends putting the best cable on the source- For me, my Ayre Codex DAC.
I have some AQ Thunder due to arrive shortly.
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Some power cords work best on amps and some on sources gears.  For example, when I put my finest power cord on my DAC or music server, it didn't sound as good as my other power cord that's made/designed for source gears.  
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Depends on equipment and cords. No definite general answer. Listen. I would start by putting the cord on the preamp first, then move upstream step by step. After that to power conditioner if you use one and then to power amps. Unless specifically designed for digital, transports and dacs, good heavy gauge cord should work well everywhere. Purist Dominus cord that I have works best everywhere ! I left it on the integrated unless making recording from vinyl to tape - then I put it on the phono stage. I record directly from phono, no tape-outs.
I have a Vitus integrated amp and separate CD player. The biggest effect was on the CD player.   A huge difference on some power cords (Shunyata especially).   On the amp the power cords had less effect, many had no effect at all. An improvement in the bass tautness was the main effect on the amp (again Shunyata was the most effective). 
Assuming power cords make a difference and a typical system of power conditioner, signal source, pre and power amps there are 16 [2^4] combinations. Double for each additional component TT, CD, DAC, EQ, DSP, etc.

Bonne Chance @@
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...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.
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
I would put it on Audiogon, sell it and use the proceeds to purchase some music. That’s just me though.
I don't have a best as they are all from the same stable and now I know the best is all round and musically the best I could afford.
On my Node 2, I use a Nordost Purple Flare with good results. Best results came when I grounded the Node 2, especially when I added the Synergistic Active Ground Block to my system.

I always had my best one on my integrated amp until recently, I moved my Swisscables Diamond power cord to my Jay's Audio CDT MKIII transport and it made a substantial improvement in SQ. I will have an equally good power cord from Silnote Audio going to my dac also. I have an Audio Envy mega3 on my integrated now and am quite pleased with the results. In truth everyone needs to try different things in their own systems to really know for sure what works best for them though.

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Right cable for the right component.  You don’t want too heavy a cable for your source, just like you don’t want too light of a cable for your conditioner or your power amps.  

Thus, “best” needs to be viewed in context of HC vs Source.  I would put your best HC cable on your conditioner.  I would experiment with source cables on your preamp, DAC and transport.  It will come down to which one had the most sensitive power supply.  

The power conditioner has my best power cord and it covers all source components.  Next are the power cords on my amplifiers.  Think about it, the power from the wall is powering your speakers.  If you have any investment in speaker cables then you should have the equivalent investment in power cables going into the amp(s).