BassDigger
500+ Head-Fier
Among aes, coax, optical and I2S, which one you think is the best?
I can't speak for any Gustard products, nor have I ever done any direct comparisons. But there should be only one winner there, that's I2S; it's the only one that includes the clocking information.
Cable runs have to be kept very short (like < 30cm) though!
Optical has the worst rep (because it has another conversion process), but will give electrical isolation (from a 'noisy' pc).
So:
I2S
AES (balanced)
Coax (spdif)
optical (toslink)
...in that order.