Which wire has biggest impact?/

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.

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

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.

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. 

@bopper
off topic but I know a guy that goes by the nickname bopper.  Do you happen to play trumpet?  Does the name Crossmen mean something to you? 

If not, pardon the interruption.
if so, we know each other.

cheers,
Bruce

The Poynting vectors don’t travel in the wire, silly. By the way, your auto spell check appears to have blown itself up. 

Which wire has biggest impact?/

a broken/ripped wire...will not transmit signals...
make sure yours are continued

Digital coaxial cables are SO different from one another that I can make my system sound VERY different by using any of the three that I own. Each has its own virtues and shortcomings.

None of the above.

slap a turbo on it.

When I first got into cables I remember switching from the top of the line Monster Cable speaker wire to Furez 12AWG cable on a pair of Legacy speakers.  Furez speaker wire only run $1-$2 per foot.  The difference in clarity and extension blew my mind, it was not subtle.  I've been a believe ever since

The wire in the fuse. It’s big. Big big. Bigger than big! 

This subject is getting old quick.

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. 

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. 

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

Get everything be a sport.

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....

A lot depends on what you have for electronics etc. The biggest improvements in my system came from Power. Dedicated Circuit, Power Conditioner, Power Cords, Fuses. I think almost everyone else will tell you that as well.