There's no one-size-fits-all answer to that, because every system will be different in that regard. Could be any component in a system. In general, all audio components in a system will benefit with isolation, individually and synergistically together. If I had to choose one component, overall, based on the odds to be most likely to benefit most substantially in any given system, I'd say the DAC. Or in an analog system, the phono stage or preamp.
Herbie's Audio Lab