Audiotroy more or less has it right.
USB is best in theory, since it simply delivers bits "in bulk" and lets the DAC sort out timing etc. Crappy DAC, crappy sound. greta DAC great sound.
All the rest are variants of SPDIF (yes, AES is a variant of SPDIF)
SPDIF has issues since in it most common configuration, the SOURCE sets timing. Your source, especially if it a computer, is unlikely to be all that great. So this is inferior to USB - all the blah, blah blah you will hear aside.
Within SPDIF optical is the worst. It has the advantage of being largely immune to noise, so very good for TVs etc - and guess what That's where its mostly used. But the most common configuration doesn't even support above 44/16
Withing "COAX" you absolutely NEED a cable designed for digital Thsi has 2 characteristics: 1) it maintains a good pulse shape, which means frequencies way above audio. 2) it has a 75 ohm characteristic impedance. Most are yellow by industry convention. Some regular RCAs are so bad they don't even work at all.
Ideally you would use a BNC connector since they can be bought 75 ohm. Al RCAs are 50 ohm. They work. They are on everything. They are sub-ideal.
Note: the small, subtle differences in sound we discuss with analog equipment generally dont apply to digital, except as they affect timing. And with USB timing is meaningless until the DAC reconstructs the signal.
Some of yoou are saying"that's BS i hear differences with USB sources". And you may - its mostly ground noise which is a big problem from PCs to DACs and any sources is a PC in disguise.
As to "forget USB". Forget that advice. Apparently a crap DAC or some other issue like ground noise. Fix it.
