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

I think having a seperate transport and DAC. Each built to the highest quality standards available.

The Transport is very important in getting the data off of the disk and getting into an electrical form. vibration control, really good lasers, power supplys are all important

The DAC is where the data is changed from Digital to Analog. To me the "music" is created in the DAC from the information that it receves from the transport. Having a really good DAC has a lot of advantages, like you can connect other digital inputs and still get a very good Analog signel out.
A DAC that uses Burr Brown DAC's and butterworth filters are among the best there is. (DAC = digital to analog converter)

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.