ND filters (also called neutral density filters) are mainly relevant when you want to achieve longer exposure times in bright light. The stronger the ND filter the longer the exposure times you can get. 
Weaker ND filters like the ND8 extend the exposure time by a factor of 8 while stronger ones like the ND1000 extend it by a factor of 1000. The right strength depends on your use case and the effect you're aiming for.

What's in my backpack?

I own 100x100mm ND filters in the following strengths ND8 ND64 and ND1000. In rare cases I even combine the ND1000 with the ND8 to achieve an 8000 times light reduction.

What shutter speeds should I aim for?

Motion becomes visible when the exposure time is long enough for the subject to move during the shot. For fast flowing water as little as ¼ second is enough to create a sense of motion. For drifting clouds depending on their speed several seconds to minutes may be required. 

Here are some general recommendations as a guideline

Which settings should I use?

In general for photos with a large depth of field I recommend using the lens’s “sweet spot” – the aperture range where sharpness is optimal and depth of field is high typically around f/8 to f/11. 

ISO should be set to the lowest native value when doing long exposures on a tripod usually ISO 100. Auto settings must be completely disabled. 

Especially with older cameras autofocus may not work properly when using strong ND filters. In that case you should focus without the ND filter first then switch to manual focus (MF) so the camera doesn’t try to refocus when taking the shot. 

Shutter speed is the variable that primarily depends on the available light the chosen aperture and the strength of the ND filter. Many modern cameras can measure exposure accurately even with multiple filters stacked. A reliable method is to first determine the correct exposure without any filters (for example using aperture priority mode) then multiply that exposure time by the filter factor and set that time manually. 

Exposure times longer than 30 seconds usually require BULB mode. In BULB you can expose for minutes by either holding down the shutter button programming a remote shutter release or using the in-camera timer settings.

Which ND filter makes the most sense and when?

Stronger ND filters are especially useful in bright conditions where long exposure times are desired. A waterfall in direct sunlight for example might only show slight motion blur with an ND64 but with an ND1000 you can achieve exposures of several seconds. 

In a shaded forest an ND8 is often enough to get shutter speeds that create pleasing motion effects. To capture the effect of moving clouds during the day an ND1000 is almost always necessary. If that’s not enough I combine the ND1000 with the ND8. Since the filter factors multiply this gives me an effective ND8000. 

Personally I'm a big fan of very long exposures in the minute range because water surfaces become smooth moving clouds gain dramatic motion and people walking through the frame often become nearly invisible. That’s why the ND1000 is my most used filter followed by the ND64 which I find to be the most versatile when not shooting exclusively in bright sunlight. The ND8 helps me extend exposure times in darker environments and also serves as a great addition to the ND1000 for an ultra-strong ND setup.

And what about ND Vario?

ND Vario screw-on filters are also a great option and offer a higher degree of flexibility. Especially for those who work with screw-in filters, this is a very practical solution. I would definitely recommend going for the stronger versions in this case.

Conclusion

I love long exposures lasting several minutes, which makes the ND1000 my most essential neutral density filter. If you shoot more often in forests or shaded areas, an ND64 will serve you well. And if you prefer using screw-in filters, you'll benefit from the flexibility of a (strong!) ND Vario. Enjoy your photography and good light!