I don't think we need "objectively best" here—merely "subjectively best". And because each person could have their own AI DJ trained on an arbitrarily rich set

I don't think we need "objectively best" here—merely "subjectively best". And because each person could have their own AI DJ trained on an arbitrarily rich set of preferences and experiences from their one-person audience, we should expect that AI DJ to be subjectively the best for their respective human. Obviously, this overlooks the shared experience element of music, but that's not so relevant in the case of online music we consume solo.