This is strictly my opinion, but I have owned many, many DACs and transports and servers.
In Order:
1. AES/EBU (if available)
2. Coax with BNC connection
3. Coax with RCA
4. ATT Glass Fiber
5. Toslink
(The above list is using a proper cable with the proper terminations and impedances).
I have never owned any gear that had Firewire or the I2S interface or any other proprietary connections. Those are "supposed" to be better.
In Order:
1. AES/EBU (if available)
2. Coax with BNC connection
3. Coax with RCA
4. ATT Glass Fiber
5. Toslink
(The above list is using a proper cable with the proper terminations and impedances).
I have never owned any gear that had Firewire or the I2S interface or any other proprietary connections. Those are "supposed" to be better.