The most comfortable sac ive ever had on my back, was one specifically designed for adventure runners by Kamelbak.
Unlike most other sacs, it had three chest harnesses, and a specially moulded contoured back, that simply clung to your back and shoulders like a limpet.
It was around 20L capacity by what i can remember, and had just enough pockets to be utilised by a person looking for extreme comfort on their shoulders.
Camelbak also gave you the option of using the hydration bladder in the sac, or removing it, for extra capacity.
Either way, you simply did not know the thing was there, and there was little or no lateral movement when on the move.