Which wire has biggest impact?/

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.

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....

Get everything be a sport.