DAC connection: which type gives highest fidelity?

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. 

+2 @twoleftears and @ghdprentice 

Building on their comments, the popularity/ubiquity of USB is pointing to manufacturers continuing to work towards optimizing it. They realize it will be most likely to be used (e.g. during reviews) and weak USB implementation will be an Achilles heel for product sales. 

Remember that I2S was originally used inside equipment and isn't standardized. Caution to do model-specific research if you're planning to pair devices with it. There are potential compatibility issues. Cheers,
Spencer

There are many variables to consider, above all the quality of the dac relative to the inputs. Schiit Bifrost 2 has a very well designed usb input that many rave about. Other dacs now offer I2s, which tends to be the cleanest/direct and most consistent digital connection…but again, depends on the internals. Types of chips, topology a s configuration, and then output stage of the dac involved. I’m personally going to be moving my steaming source from coax to I2s by connecting it to my recently purchased Topping D70s. This dac is pretty exceptional for the price, already sounding better than my previous through every single connection (haven’t tried the aes yet).