For me, its the first power cable from the wall to the power distributor. I've tried a few power cables, but all in the reasonable to entry level price range from Kimber, Audioquest, Pangea and Triode labs. They all did small things differently, like tighten the bass here and there. But nothing big enough to justify the expense, till I tried an Ansuz Mainz A2. This thing was a very noticable change, and head and shoulders above the rest. And the noise floor of the entire system did drop noticably, with just one cable from the wall to the generic distributor Iam using. This reduction in noise, made everything stand out more. The added resolution, and quiteness made the smaller details seem easier to pick out in the mix. Like the reviewers favourite phrase, I think I was hearing new things in familiar tracks. The bass became the tightest, most impactful and hardest hitting I've ever had in my room.
At any point in time, it takes atleast 10 power cords to get the system running, as it is composed of all separates. Without touching anything anywhere else, such a big change was wrought by one single cable, placed at the head of the chain.
This made me a convert to the Nordost theory of cabling heirarchy. Which starts with the very first power cable from the wall, so that the benefit of filtering provided by this cable, perculates to benefit every single component down the chain. So the best cable should be the first up the chain, be it power or interconnects. My best interconnects connect the Dac to the preamp, and next best from preamp to power amps....and so it goes.
I found the least benefit with interconnects and speaker cables in my system. The differences were tiny for the huge asking price. But then all these cables are at a similar price and performance range. So Iam not suprised. Your mileage may vary of course.