For the best night's sleep, back sleepers should be sleeping with a slightly raised head. Medium support pillows are the best pillows for back sleepers. A pillow with medium support helps to prevent undue pressure for your neck without causing your spine to bend lower down.
So whether you sleep in the Starfish position (on your back with stretched-out arms and legs) or the Soldier position (on your back with the arms at the sides), a medium support pillow for back sleepers is ideal.
If you switch between your front and your back, use two soft support pillows so that you can use them both when you're on your back and discard one when you're on your front!