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.
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.
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.
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!
105MM POLARIZER RING
This ring mounts on the filter holder and the polarizer screws into the ring.
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.
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).
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.