What's the best (or is there a "best") DAC input to use?

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).

You're welcome.  At the beginning USB was used as synchronous meaning D/A converter clock was derived from unstable computer data rate (to avoid loosing or gaining too many samples).  Async USB should be better.

Be careful with "better" transports.  Some of high end transports have fast transitions, in order of 5ns, making cable impedance match critical.  In such case using very short wire (that is not considered electrically "long") is pretty much impossible.  Rule of thumb says cable becomes transmission line (reflections) when its propagation is longer than 1/10 of transition time.  1/10 of 5ns would be 0.5ns equivalent to 10cm of wire (assuming 5ns/m) including connections inside of transport and the DAC.  On the other hand fast transitions reduce influence of electrical noise.