Toslink

If I remember correctly, Rob Watts (who designed many of the recent Chord DACs) likes optical/Toslink more than many of the other connection types. Most other designers in the hifi space seem to rank it last in terms of sound quality. So who knows. 

@soix 

For digital out from the Audioengine B1, it is Toslink only.  Period.

I don’t have a choice.

Plastic Toslink cables are for the masses and will (most likely) degrade the sound. GLASS Toslink cables can sound very close to a wired S/PDIF digital connection.

Toslink is only a benefit where the galvanic isolation is needed. It relies on light bouncing off the inside walls of the fiber as it travels so yes glass is most definitely better. Whatever transmits light better. 

Different sounding Toslink is just different levels of data loss, jitter, degradation.

Always use USB or I2S whenever possible.