USB theoretically should be the best as long as it is asynchronous. As name suggest D/A converter clock is completely independent from the computer. Data is sent in chunks (frames) while DAC sends back signal "Buffer over/under-flow" used by computer to adjust size of the frame. Of course USB cable can inject unwanted electrical noise especially when DAC is powered form USB. It is desired to use cable without power wires, when DAC has own supply, but even then there might be some amount of digital noise on the cable. Still, I would start with USB first.
Choice between Toslink and Coax is not so simple. In most cases Coax should be better, but it is system dependent. Toslink, being optical, cannot create ground loops, but has slow transitions. Slow transition thru threshold point combined with system noise can create variation in level recognition moment - a time jitter (noise in frequency domain). Faster transition in coax can cause reflections on characteristic impedance boundaries, that can modify (add to) shape of transition thus causing jitter, It is a function of all impedances and expensive cable won't necessarily be better. What works in one system won't work in another - you need to try. Pick at least 1.5m long coax cable (reflections will come too late to add to transition) or very short cable <12" (no reflections).
Choice between Toslink and Coax is not so simple. In most cases Coax should be better, but it is system dependent. Toslink, being optical, cannot create ground loops, but has slow transitions. Slow transition thru threshold point combined with system noise can create variation in level recognition moment - a time jitter (noise in frequency domain). Faster transition in coax can cause reflections on characteristic impedance boundaries, that can modify (add to) shape of transition thus causing jitter, It is a function of all impedances and expensive cable won't necessarily be better. What works in one system won't work in another - you need to try. Pick at least 1.5m long coax cable (reflections will come too late to add to transition) or very short cable <12" (no reflections).