Use the ND filter which allows you to achieve the most appropriate shutter speed for your scene. If you are unsure of what shutter speed would give you the best result for your image, give yourself some time to experiment with different filters at any given scene. These are some basic recommendations which you can modify to suit your own style:
ND8 | 3 Stop
A three stops ND filter is good for smoothing out dynamic water while retaining some texture. This suits waterfall or seascape shooting. Your target shutter speed is generally between 0.3 to 2 seconds.
ND64 | 6 Stop
A 6 stops ND filter is what I would generally use during golden hour in order to create exposures of 1-2 minutes for smooth water or clouds. A 6 Stop is also great for capturing dynamic water while retaining some texture before the golden hour.
ND1000 | 10 Stop
A 10 stops filter is used during the day outside of golden hour as the bright ambient light requires much more ‘light blocking’. These can prolong exposures for up to 5 minutes during golden hour with the appropriate settings.