Hope that helps!
HDCD are/were made with dynamic range compression and uses dither with a special subcode embedded in it that controls filtering and a "expander"
the dynamic range compression is mild and quite listenable without the HDCD processor - as a design goal
the HDCD reciever chip/sw also upsamples allowing "room" for a few different digital filter characteristics - I don't know if this feature was often used
likely the HDCD processing tools were only bought and used by studios/engineers committed to superior mastering practices in the 1st place
I guess a normal CD. But the marketing department will tell you that it's made from unobtanium and polished with tears of the last unicorn, to improve storage quality of bits ... 101110 becomes 101110. Don't you see how much clearer the latter is?