This is such a difficult question to answer generically.
First, each DAC implements its inputs differently: some just seem to favour USB, coax or - even - optical.
Second, cables matter: a poor coax often sounds worse than a bog standard USB cable, and cheap optical cables sound awful - from which much of its poor reputation springs. Audio grade USB cabling is a science in its infancy.
Third, the PC-side implementation matters, and varies: USB from my Mac Mini, for instance, sounds conspicuously worse than USB from my MacBook, even when the laptop is powered.
Fourth, USB->SPDIF conversion is often lossy and depends on the quality of the converter. The money you need to spend on a top-notch converter would more smartly be spent on a better DAC.
What I can tell you does work is the VdH Optocoupler which, for $60, completely changed my opinion of optical connection.