Where do cables upgrades have the most impact?

Assuming all existing cables in a system are atleast mid-grade (not junk), which cables/interconnects should be upgrade first and in what order? Where should one start?
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Start preamp,line conditioner then everything else do this ASAP!!!!!
@djones51, that was hilarious  :-)
Curious if those saying the power cable had the most impact are plugging into a power conditioner or a wall outlet?
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Looks likes theres no consensus . Best bet is to buy a cable and try it out on all your components .