Where do cables upgrades have the most impact?

IME, power cords and interconnects have the most affect on sonics.
I like to start an upgrade with interconnects since they influence the timbre and sonic signature the most. They tie the components together. I start with the source.

My power cords are upgraded depending on the strength or weakness of each component. Often the most noticeable upgrade will be the power cord coming from the wall.

Most online cable dealers offer a trial period included with a sale.
So, the next step is to decide on which brand to try.

I would advise introducing one cable at a time when auditioning.