So, nothing anywhere near approaching a “best” or even an almost good DAC.
I2S is a perfectly fine way of moving data over carefully designed PCB traces inside a cdevice, where all components are controlled by device’s internal clock. Using things for what they are designed for and all.
When used as an external connector, I2S, apart from allowing for higher bit rates, is about the same as S/PDIF, requires more electrical conversions than USB, brings jitter back into the picture, and creates all kinds of interoperability problems because… oh, right, because it is not a proper inter-device connection, and nobody is going to standardize it, like S/PDIF, AES or USB are standardized.
Ethernet is a perfectly fine way of moving data. If you are moving it over a network. If you are not, it does nothing. From the practical standpoint, on anything that was designed by engineers and not marketers, after data has been extracted from either Ethernet or USB packets, there is no meaningful difference in either data (obviously) or electric noise. No reviewer worth reading had ever either heard or measured any. There’s no reason for it to be there, unlike I2S that can show more jitter, depending on implementation.