In my humble experience a ND filter works best when it comes to slow down the speed of the water. In such low light levels with an aperture of f16 and assuming the filter factor is about 4-6 stops the results should not be a disappointment.
A circular polarizer can also do a good job although it is only a 2 stops filter factor. That should not be an impediment if using a very slow ISO setting considering the camera in use is capable of such setting.

The polarizer will work but it will never do a better job than a ND filter. Where the polarizer will shine will be in eliminating or reducing the reflections that are usually present on the stones or leaves around the fall.

Of the images you have presented to us you will have a variety of different answers because we are all different and we have different tastes. To my eyes some of those images would benefit from using the portrait format while others will benefit from cropping non desirable surroundings. I prefer No. 2 and I will suggest to correct for a light cyan color cast present in all of them, easily done adding equal parts of red and yellow using the Color Balance of your editor.

Shutter speed for waterfalls should be chosen by what you like and to some extent the waterfall itself. Some people like the ultra-silky look so do I sometimes. Some people like the frozen water look, so do I sometimes. But normally I go for something in between. I use a polarizer and ND filters because I sometimes come across a falls when traveling that I hit at the wrong time of day.

My only caution is that if there is a lot of mist. And I do mean a lot. Faster shutter speeds can be better. Otherwise, you just end up with a gray/white blur. I found that out the hard way. I shoulda chimped a little.