"A Must have" ND filter stops for filming with Canon 5D Mk2?
There is no "most used".  You use whatever is appropriate for the exposure conditions you have and the desired aperture and shutter speed (and ISO, though turning down ISO is almost always better than using an ND filter if it is an option).  The more light you have, the more wide open the aperture and the slower the shutter speed, the heavier the weight of the ND filter you need.