My vote goes to the Acoustic Revive cable on Sp/DIF and while I have experimented with Siltech, Chord and others, I feel that on the Acoustic Revive replacing the wallwart with a linear PSU yields real benefits. I think there is a need for research on the transmission of clock signals in digital links which are paramount for good results and Acoustic Revive's circuitry majorly contributes to accurate clocking.
USB is a different topic, altogether
USB is a different topic, altogether