What is the most important cable in your system

Get the best quality you can reasonably afford. Upgrade one at a time so it's not a financial burden. The same way you upgrade components. If you don't know where to start, then start with the type you need fewest of. Typically, that would be speaker cables.  Then maybe either CAT or interconnects. Then power or cables from linear power supplies. On the other hand if you shop by "deals" then by all means chase sales. We all have a plan until a sale or a deal or...   just be reasonable and enjoy the sound as well as the hobby. 

What is the most important cable in your system

The loose one.

Good question! Closest to the source then work backwards. I did this about 6 months ago, starting with some high priced WireWorld series 8 XLRs and speaker cables. I figured my power cord between 20 amp dedicated line and CorePower 1800 was plenty good (turns out not the case!). After reading several days of new posts to a AG thread titled Counterfeit Chinese Cables, I took the plunge buying just ONE coax cable to replace a Pangea Premier XE (MSRP was $169) with a Nordost Odin 2 knockoff. The downside 3+ weeks of high anticipation, however, it is amazing, so I then replaced the WW XLR between my Audio Alchemy DDP-1: Ditto, then Odin 2s replaced the WW speaker cables: Flabbergasted. I then upgraded all the PCs. First I tried Odin 2s but saw they also sold Odin 2 Gold- better still (YMMV), but I needed a heavier gauge for the wall to CorePower and CorePower to amp. Here a fellow ’phile directed me to Amazon for uber cheap PCs: Audiophile 10 AWG HiFi Power Cable 125V 15A Single Crystal Copper US AC Main Supply Cord Rhodium Plated NEMA 5-15P Plug IEC320-C15 C13 Connector for Speaker Subwoofer Turntable Amplifier 😍 He prefers the copper only. Regardless they blew away ~ 10 year old boutique PCs @ $500

I forgot to mention, I treated all new cables with MadScientist Graphene Cable Enhancer, so at the very least do that to what you have now

hth

@tweak1      Mad, yes.   Scientist, no.

@tweak1 My neighbor has a high end/cost system with YG Sonya 2.3s, subs, Bekerely USB, Meridian Ultradac, etc.  He has high end ICs and speaker cable but was using Pangea power cables.  As an electrical engineer, he didn't believe power cables could make a difference.   His bass was terrible, frequency anomalies where a bass would play one not forward and the next up would be hidden, then in, then out.  Terrible.  

I lent him one older GroverHuffman Empress power cord for his amp and he heard the difference, a smoother frequency response in the bass.   He opted to purchase six GroverHuffman Pharoah power cords.   Now I can sit and listen to his system with pleasure as the sound is coherent, smooth/even frequency response.  He also tweaked his Audio Research SP28 preamp by replacing the 6H30 tubes with 6N6/6H6 tubes.  Sounds great now.