"What would you rather people said instead of 'couldn't care less ?'"
nothing. couldn't care less is perfect. it makes sense. On a scale of caring you are at the absolute minimum point of caring.
It is americans who say "I could care less..." which gets my goat, because it makes no sense.
I have heard it originated from exchanges between british and yank troops during the war, but this may just be an urban myth style explanation.
So you'd get
brit soldier: I couldn't care less
American soldier: oh yeah buddy well I could care less (i.e. I care even less than you do about this)
and in that situation it has at least a semblance of sense to it.
As a stand alone though it is just bizarre
person a: well I think xyz
person b: well I could care less what you think.
absolutely no logic to it