Raspberry 4: best i2s dac?

Not sure I understand your question.

Meaning USB dac compatibility list.

Did you search on the forum?

Yes. N/A. Isn’t there any common features, e.g. DAC AK4493, XMOS XU2/3** etc. having certain default values?

Nope, We support Volumio not all DAC brands.
So we (read community) report DAC’s that work or don’t work with Volumio.
It’s your task to determine what your DAC needs to do and find one covering your requirements.

The majority of standard USB DAC’s will work with Volumio