What is the best place to put a dehumidifier?
Humid air rises and warm air that can carry more moisture rises.  Cooking adds moisture to the air.  All else being equal, the dehumidifier has a easier job removing moisture from cooler air with a higher relative humidity.

I would place the unit upstairs in the central location with good air circulation, nearer to the kitchen and away from any heat sources.  The location also needs to be consistently warm enough that the dehumidifier will not collect frost or cycle off on its low temperature limit.