Digital or Analog. Let me be clear on one thing though, I am assuming that what sounds good also has good measurements. There are actually many extremely high end components that ADD distortion to get a certain sound (Tube Amps do this). The audiophile likes this sound and then claimes it to be "Better". I believe that components should be tranparent and any colorization should come from the recording engineer. If we assume that by good sound we mean it will also have excellent measurements than you can apply the following rule to electronic components:

Elecronic componentes will be audibly indistinguishable if they have:

1- Flat Frequency Response
2- Noise and Distortion levels below audible thresholds
3- High Input impedance and low output impedance

Now this is the engineering/scientific and measurable point of view. Many people claim to hear things that are NOT measurable or claim that non measurable things still affect the things we can hear or that "Good" sound does not measure as such. Thats where we go off into exotic audiophile land with $1000 speaker cable, CD rings, and wierd amp, pre-amp designs, etc.