DAC connection: which type gives highest fidelity?

I would suggest optical network-based raw data transfer (e.g. a UPnP renderer) as it has both error-detection and retransmit while also being electrically decoupled.

But, in practical terms, it pretty much depends on the implementation of your sender and receiver. For some devices, the USB input may be better than the coaxial one, while for others it will be the opposite.