I use the Formatt-Hitech brand filters. I continue to be very happy with the quality and durability of their filters, rings and holders. I selected to use 100x100mm filters to help future-proof myself if I buy a wider diameter lens in the future.

 
 
Formatt-Hitech Firecrest ND 1.2 (4 stop) ( Amazon )

Formatt-Hitech Firecrest ND 1.2 (4 stop) (Amazon)

Firecrest 4-Stop ND

This is my go-to filter in the field. Cutting 4 stops of light usually gets me that silky look on the water I'm looking for, while maintaining detailed fingers of water.

Formatt-Hitech Firecrest ND 1.8 (6 stop) ( Amazon )

Formatt-Hitech Firecrest ND 1.8 (6 stop) (Amazon)

Firecrest 6-Stop ND

When 4-stops isn't enough, I have the 6 stop at the ready. It's also very useful when the best light is just after sunrise or just before sunset, when I need to cut more light.

 
Formatt-Hitech Firecrest ND 3.0 (10 stop) ( Amazon )

Formatt-Hitech Firecrest ND 3.0 (10 stop) (Amazon)

Firecrest 10-Stop ND

For extra-long exposures, I keep an 10-stop ND filter in my bag. I use it to smooth out the ocean or to make people magically disappear in busy areas.

Formatt-Hitech Circular Polarizer ( Amazon )

Formatt-Hitech Circular Polarizer (Amazon)

Firecrest 105MM Circular Polarizer

No landscape kit would be complete without a circular polarizer. I use my polarizer to manage relfections in a scene. It also acts as a light ND filter (1 stop or so).

 
Formatt-Hitech Filter Holder ( Amazon )

Formatt-Hitech Filter Holder (Amazon)

100MM Filter Holder

I keep two filter holders in my bag. One has my polarizer permanently mounted, with an open slot for an additional ND filter. The other I have setup with two slots for ND filters. I could have three slots, but never found the need to stack up that many filters!

Formatt-Hitech Adapter Ring ( Amazon )

Formatt-Hitech Adapter Ring (Amazon)

Adapter Ring For 100MM Holder

Adapter rings come in a variety of diameters to match your lenses (ex: 67mm, 72mm, 77mm). I have a ring for each of my lenses. The filter holder mounts to the adapter ring.

 
Formatt-Hitech Polarizer Ring ( Amazon )

Formatt-Hitech Polarizer Ring (Amazon)

105MM Polarizer Ring

This ring mounts on the filter holder and the polarizer screws into the ring.