Which wire has biggest impact?/

Trying to upgrade my cables- If I wanted to try a new cable which one would have the most impact-I know they are all important but which one would you start with- speaker cable- interconnects etc- thks
The biggest improvement will be replacing the one that's the worst. If they are all freebie rubber power cords then one of those. If they are all freebie patch cords then one of those. If you're using speaker wire sold by the foot then replace that. If all of the above, sorry, I darn near blacked out from the horror....
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.