Which wire has biggest impact?/

Millercarbon got it right - whichever one is the worst. 

If they're all the same, I'd start with the interconnect between the source and the preamp, then the preamp to poweramp, then speaker cable. 

i agree with @millercarbon.your system is as good as the weakest link in the system.find the weakest link or component and upgrade it suitably.

My take:
Speaker cables, Power cords, interconnects, ethernet cable, power conditioner in that order. 

The general recommendation is to spend 10% the cost of the entire system on cables. This would include the speaker cable, interconnect, power cord, USB, digital and other types of cable.

Personally I have spent about 25% the cost of my entire system on cables. In my experience, speaker cables and interconnects are equally important. Power cords, not so much. I will be upgrading the USB cable very soon and will see if the difference is worthwhile for the price paid.