if your good like someone said like reinforced card board you can use your imagination like you can roll up cardboard and swing it or you can trap people in boxes for a little time so you can run or sneak attack cardborad has so many more options that the pillow fight feathers
I'm going with feather because not only is there a sharp tip that's going to be useful for stabbing, but there is also the element of biological warfare, i.e. give your opponent avian flu.