In my experience, the answer to your question mostly depends on how the associated equipment is optimized. I have used the USB, Coax, and optical connections that you asked about, plus AES/EBU and I2S. In general, I2S was intended for very short runs inside of equipment and not as an external connector for two different pieces of equipment. My current main system gear is set up to optimize USB so that is how I use it. However, I use optical for my less expensive garage/outdoor system (Metrum Baby Ambre to Jade) and do not perceive any loss of SQ. In cases where all connections were assumed to be equal with the gear being used, I have mostly preferred AES/EBU. When using coax, I have mostly used BNC (and not RCA) connectors for reasons quoted below by the owner/designer of Empirical Audio.
I have explained this many times on this forum. RCA connectors cannot be properly terminated to coax cable, I don't care what RCA you use. Coax cables have BNC connectors specifically designed for each type, so that the impedance discontinuity is minimized. Using RCA terminated to a coax will cause unwanted reflections, resulting in jitter at the receiving device. Adding a BNC to RCA adapter is usually harmless and does not result in compromised SQ.
Steve N.
Empirical Audio
In summary, try the different connections and use what sounds best to you.
