Most Beneficial Cable Upgrade

No, because there is no answer, it depends on the perticular system in question.

AC power cords and most other power "conditioner" products. The sales claims sound logical: Noise and static can get into your gear through the power line and damage the sound. In severe cases it's possible for power-related clicks and buzzes to get into your system, but those are easily noticed. The suggestion that subtle changes in "clarity and presence" can occur is plain fraud. Indeed, every competent circuit designer knows how to filter out power line noise, and such protection is routinely added to all commercial audio products. Spending hundreds of dollars on a six-foot replacement power cord ignores the other hundred-odd feet of regular wire between the wall outlet and power pole.The real answer is? Use logic and really listen with someone else but you doing the switching and you might just see the silliness of it all.