DAC connection: which type gives highest fidelity?

It depends. Generally optical is not good, skip that one. But USB versus SP/DIF or AES depends on the DAC since USB requires the DAC to retime the signal the others do not. So, it depends on the quality of the DAC versus the source. Generally, AES or double AES should be better than single ended SP/Diff.

I2s and Ethernet. USB is the worst

as usual @twoleftears and @ghdprentice get to the heart of the matter

My understanding is that different manufacturers have optimized different connections among the available options. So it's really impossible to generalize about.

Yep, end of discussion.