What is needed to have the best CD/Digital sound?

Tubes and good recording cd's.

A high quality remaster is the most important thing, sometimes we lose sight of that in the quest for more gear. Then a good source is needed, and I disagree with Mark above, there are just as many good one box solutions now as there are dac transport combos. Lastly it certainly doesn't hurt to have a tubed preamp amp or both.

Tube amps and preamps, a DAC or player with tubes in output section will bring even more warmth and fullness.

you need a well recorded and mastered disc and a machine capable of extracting what's on that disc. btw i also dissagree with Mark02131 about the need for two boxes. that may have been true in the past, but not any more.