Which digital output is best? USB, S/PDIF, AES/EBU?

Every DAC is different, and every system is different. So, there's no easy answer to this question. USB vs. SPDIF/AES/EBU may sound slightly different from one another. But which one is "better" is entirely subjective. It really depends on the DAC and personal preference. In many cases, USB will offer the highest sample rate/DSD compatibility. Other inputs may be limited to lower resolution playback.

Due to the nature of asynchronous USB audio connections, in the case of USB connections, the DAC's clock is in control of the flow of information. However, with W20, W20SE, N10, and N20 the SPDIF and AES/EBU outputs are driven by a best-in-class "OCXO" clock which results in ultra low jitter. It is one of the most precise clocks available for audio. So, if you have one of those models, you really ought to at least try using something BESIDES USB! You may be in for a major treat...