On wall near floor or ceiling for supply ducts in basement
Ceiling.  Basements get water on the floor.  You do not want your air quality messed with based on a leak or flood.

Also note that if you have a properly insulated basement that its temperature should stay relatively calm compared to the rest of your house so the heating and cooling needs are not as impactful as they are on floors above ground.  Meaning if your basement is done right you should need very little AC in the summer and very little heat in the winter to keep it comfortable.  (I have three vents in my basement and they are always shut.  In the winter if I open them it is literally hot as hell.  In the summer they only are needed after we air things out.)