Which wire has biggest impact?/

Get everything be a sport.

this issue has been discussed many times, check the archives....

No one can answer that question; too many variables. Put aftermarket cables on ALL connections for superior sound. Includes power, IC, SC, umbilical's on power supplies, USB and Ethernet. 

If your components are lower end, use the manufacturer supplied power cords for now. Do interconnects and speaker cables and upgrade components/speakers. Power cords and better IC and SC will come with time. Do not fail to eventually also upgrade USB or SPDIF (coaxial) and if you wish, Ethernet. 

My opinion would be...
If you’re running single-ended, then the upgrade the interconnects between the preamp and the amp, or source to integrated. If you’re running XLR interconnects, then upgrading Your speaker cables can make a nice improvement. Followed by power cords and power conditioners. But again, just my take on it. 

This subject is getting old quick.