Digital Audio Output Differences? Is USB Audio Really The Top-notch?

I have a RME adi2 DAC , it’s a great sounding DAC to be sure....   I use all three inputs and honestly I can’t tell the difference between my coax and the USB.  It sounds so good from all three , it’s like splitting hairs.   

I2s is the best connection in my limited digital experience but finding equipment from different manufacturers that will have the same I2s configuration is a challenge. I don't stream but tried connecting the ps audio pwt to a gustard x26 pro to play my CDs with zero success. Now I switched to the Jay's cdt2 mk3 and the I2s connection to the gustard is fully operational. The sound is frankly so close to my analog rig now, it's hard to tell the difference. And I have twice the dollar amount invested in my analog rig vs. the digital one. I am completely sold on I2s when feasible. And I also want to take this opportunity to thank the digital audiogon community who steered me in the path of my digital journey. Without it I would still be mainly analog. And CDs are so much less expensive. Today I ordered $120 of CDs on discogs including shipping and I am getting over 10 CDs for it, about a third of what vinyl would have cost me and some titles don't even come on vinyl. This is a huge step forward for me. Sorry for getting off track here but just got through an unreal quality CD listening session tonight and felt like sharing.

USB is the worst connection you can use, even if you buy into all the gimmicks/tweaks you can buy to try to make it better.

Ethernet and i2s are much better, but only your better dacs include them and they are usually more expensive 

It really does depend on the implementation, but I have never had good results with USB. I have tried USB directly from my PC, from an audiophile server (an Antipodes), and from a number of streamers - Lumin U1 mini, SOTM, Ultrarendu and OpticalRendu, and currently my Bricasti M5. I have used good USB cables - Furutech and Shunyata. I have also tried a number of very good dacs - Chord DAVE/M Scaler, Mytek Manhattan II and currently my Nagra HD Dac, all of which have very good USB implementation.

I always find USB sounds hard, "digital" and unsatisfying. To my ears, SPDIF and AES sound more natural and tonally more complex. I currently use AES from my Bricasti M5 to the Nagra dac. However, a well-implemented ethernet input can also sound extremely good, and was the best input on the Mytyek Manhattan II.

The only way you will know is by listening to them all. But despite its theoretical advantages, USB always seems like a poor choice for audio in practice.

I have a PS Audio Direct Stream DAC.  I've used USB, Ethernet, and i2s with it.  i2s is by far the better option.