Which type of cable sounds best for Streamer to DAC: AES/EBU or DIGITAL COAX or USB ??

My opinion is that there is no one right answer. It all varies with the components you are looking to cable together. So for one set of components AES/EBU may be the best way to go, for another set USB, and for yet another Digital Coax. And that doesn’t even get into the fact that even within a given class of cables there can be a considerable degree of variation, as the cable itself is also a component.

That being said, my starting point would be to contact the manufacturer to get their take on their component. They might be able to provide some valuable insight into what works best with their particular component.

In your system, I would stick with Digital Coax between your Node and Auralic DAC. I own Vault 2, both RCA and Toslink sounded pretty flat to my ears.

As said, it depends on the components involved. You can ask the manufacturer or just see what sounds better to you.

USB has the advantage of handling higher sample rates. SPDIF or AES3 are usually limited to 192 kHz. USB does much higher. (It doesn't matter to me, but it might to you.)

Could one use an HDMI? I have an HDMI running from my source, whether it be my phone, tablet, or MacBook, to my oppo.

No idea on the HDMI, but when one considers the work put into the AES/EBU, coax and usb, with regard to taming sources of jitter and whatnot, on those three formats..

..and that HDMI is a complex system, more complex than the others and exists purely at the consumer level....

...and has had no jitter and noise taming for audio, in the high end direction...that no devices, chips or schemes to tame high end audio problems in HDMI exist...well..it clearly marks HDMI as the inferior audio data transfer format. all for a plethora of issues and potential issues, that are not easily detailed in simple posts on forums.