DAC connection: which type gives highest fidelity?

Suppose one wants to connect two digital devices, as in a streamer to an outboard DAC or similar.

Which is the highest fidelity connection?  USB, coaxial, or other?

You will have to listen for yourself. Every manufacturer has a different presentation with there inputs/outputs. Also, not all formats may be available on each output so you may not have a choice. You may need to use several types of interconnects to listen to various formats.
USB async would be the highest fidelity but for the fact that USB sources are often very noisy. Isolation is key and not all USB interfaces have that. Coax normally does though, but it'll be higher jitter than async USB.
The best is probably up for debate I2S, AES/EBU, etc. The worst, in my experience, is toslink. Your mileage may vary. 
My understanding is that different manufacturers have optimized different connections among the available options.  So it's really impossible to generalize about.
Many high end mfg are combining R2R, multi bits 
or customs dac chips with a high end streamer which inherently has lower noise when properly designed and 2 less cables. 
Bricasti, DCS, Aqua , are just a few high end companies doing this.
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.
Post removed 
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,
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).
Reason I ask:

Considering a Denafrips ladder dac (probably the Pontus II), into which I would run a Node 2i.

Want to know what I'm doing and what to expect before I jump.



a word of caution to you

the dena dacs’ input card has issues receiving a hi res signal (above redbook) from the bluesound node/node 2i (pontus, ares 2, gaia)

it is the ONLY family of dacs that has input compatibility issues with the node digital out... and i have had 40+ dacs in my system since january 2020 (no exaggeration)

alvin says the digital signal output of the node is 'on the low side', and the input receiver of the gen 2 denafrips gear thus won’t recognize it (you get a bunch of subtle pops coming through the system as opposed to music when this happens) - alvin says ’just use the optical connection’

so be warned...
WOW my computer to DAC connection is schizophrenic. My older DAC does have USB, It's a CAL Alpha.  My newer Mini Mac does not have Toslink/Fiber.  I use a USB to Coax Adapter.  Sounds great if you considering the age of my gear.
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.
Ethernet!    Data send in packets, ability to resend data if if errors, data integrity from IP layer, simpler implementation with  synchronized data transmission ...

bobbydd OP
DAC connection: which type gives highest fidelity?

The very best was/is?
If you have the optical AT&T bnc type high speed optical glass fibre connector’s that Wadia and ML’s some used in their top models, these are the best, and you use them with a dab on the ends of the expensive "optical indexing fluid" that gives them an air free mating connection. The glass fibre lead is very fragile and can't be bent too much as the glass fibre cracks.

Cheers George
This is from the Pontus 2 spec list on the Denafrips page. I have no reason to doubt what Alvin passed on to jjSS49, so beware there.
DSD64 (DoP) on Coax/AES/Opt Input  
DSD1024* On USB & I²S Input Only
24bits / 44.1, 48, 88.2, 96, 176.4, 192KHz On All Input  
1536kHz* On USB & I²S Input

Clearly, Denafrips has put a great deal of effort into their USB and I2S inputs as they probably reasonably expect those to be the most commonly used.

As have Antipodes with the USB output on their line of server/streamers of which I'm a very happy owner.

I have the S30 and use Roon with a Curious Cables USB cable into my Pontus and it sounds fantastic to me when streaming Redbook and higher.

I'm considering trying one of the Denafrips reclockers to use the I2s input as their DACs respond very positively to that input by all accounts.
Just curious about the Node/Denafrips issues. I feed my Denafrips Ares II via the Node (2021) using only digital RCA coax. I listen to Tidal and Qobuz at all bit and sampling rates available; I’ve never witnessed this at all to be honest.

I’ve seen this issue documented here and on other forums, but maybe the issue was resolved with newer generation of Node products? Or am I just lucky? 🍀
to my understanding the older models actually were ok, it is the newer models that have this problem (by the time i experienced the issue my ares 2 was long gone, so cannot comment there)

for example, my original pontus made in 2019 received the node digital feed fine, yet the newer model with ’ii’ elements (production mid 2020) as well as the gaia (both using the same input receiver) have had this problem - i actually picked up a gaia to try to solve the problem but alas, no dice...