That is a really tough question because it is so situation dependent. It depends on the look you want to achieve and also the light that you are facing. You almost always have the wrong filter with you. When I started using ND filters I always went for a 10-30 second exposure just because I was excited to get the smoothest water flow in my images. Therefore my ND1.8 (6 stops) wasnt enough sometimes. I often ended up shooting at F/22 to get the desired shutter speed.
I use the following rule of thumb: At a bright cloudless day I get an exposure of around 1/250 & F/8 at ISO 100 without any filter. On a day with thick clouds and no blue sky visible I get around 5 stops less light. The sun in Iceland never goes that high so I would subtract another stop from that for very a cloudy day. Then we are around 1/4th of a second at F/8 and ISO 100 (since one stop of light doubles or halves the shutter speed). Of course this is just a rule of thumb and the light may vary but one to two stops difference can easily be compensated. With an ND1.8 filter we could take a 15 second exposure in this situation. Sounds perfect to me. However things change a bright sunlight. Therefore I added a ND2.7 and ND3.6 filter to my kit (where the ND3.6 is too extreme in most cases).
A closer look at this table might help as well:
||Real-World Situation for Proper Exposure
||Nighttime landscape under quarter moon
||Aurora borealis of moderate brightness
||Nighttime landscape under gibbous moon
||Nighttime landscape under full moon
||Nighttime snow or beach landscape under full moon
||End of blue hour
||Late in blue hour
||Middle of blue hour
||Distant cityscape at night
||Indoor scene lit only by dim window light
||Floodlit monuments or fountains at night
||Typical artificial indoor light
||Bright indoor lighting
||Fairs and theme parks at night
||Bright window displays and advertisements at night
||Nighttime sporting events under bright light
||Moment after sunset on a clear day
||Daylight on a foggy day
||Moment before sunset on a clear day
||Typical subject on a bright, cloudy day
||Typical subject on a day with hazy sunlight
||Full sunlight on a cloudless day, typical subject
||Full sunlight on a cloudless day, bright subject (i.e. the beach)
||Full sunlight on a cloudless day, highly reflective subject (i.e. snow)
Every EV is one stop difference.
I want to quickly say something about style. After 10 years of landscape photography things have changed a little bit for me. As I said before I almost always tried to go for a 10-30 seconds exposure. However I meanwhile try to go for 1/10th of a second in most cases. Let me show an example:
The first image is shot at 1/6th of a second and the second one at 30 seconds and I much prefer the first one since it much better expresses the rapid flow of the water while keeping some characteristics of a long exposure. Im totally happy nowadays with my ND1.8 filter. In bright sunlight I sometimes wish I would have brought the 2.7 with me but thats how it always is in landscape photography. The best gear for the situation was always left at home.
As an extra tip for shooting waterfalls is to have a 4 by 4 inch plate of acrylic glass in the backpack. You can get those cheaply in every hardware store. Waterfalls tend to sprinkle the water all over the place. The air is very humid and you will be constantly cleaning your filter. I use a small sheet of acrylic glass to hold in front of my filter as long as im framing the shot. I then remove it to take the image quickly and put it in front of the lens immediately after I took the shot. Otherwise your image might be full of water droplets on your filter like this:
To clean the filter I would suggest to use an air blower rather than a cloth since the cloth is soaked after a handful of wipes but I always have both with me. Enjoy your trip!
Addition: I use Haida filters and I strongly recommend not to go for a variable ND filter since they often disappoint in quality at their darkest setting.