If you are going to get a filter holder system we at Mountains to Sea Workshops recommend either the Lee Filter Holder System or the Formatt-Hitech Filter Holder System. Both have great build quality and provide a great base level with maximum flexibility and reliability to get started capturing
long exposures. Note that you can not use these base systems for bulbous front lens elements like the Nikon 14-24mm or similar.
If you want to get a kit this is a pretty decent kit to get started with Formatt-Hitech.
We highly recommend a polarizer if you don’t have one. Not only are they great for a little bump in color and reduced glare, but they also can provide 1 to 2 stops of light reduction.
We really like the super thin B+W XS-Pro which is so thin that it has no edges showing even at 16mm.
Once you have a holder you can use 100mm square and rectangular filters
We recommend a 6 stop for a basic long exposures.
Lee Little Stopper 6-stop (1.8) ND
Formatt-Hitech 100×100 Resin ProStop IRND 6 (6 Stops) (Get 100mm at 1.8 Density)
We also recommend a 10 stop for really long exposures.
Lee Filters Big Stopper 10-stop (3.0) ND
Formatt-Hitech 100×100 Resin ProStop IRND 10 (10 Stops)
You can also get a 2 stop or 3 stop hard or soft edge ND Grad. Hard means a hard line and soft is a more gradual gradient.
I started with a 2 stop hard, which works great for the ocean where there is a hard deliniation. In the mountains a soft edge works better.
One other piece of equipment that I particular like is a filter holder for all those 100mm filters. I use the Clik Elite square filter valet seen below. It’s a simple and affordable way to maintain control over your filters. The padded sections hold seven 4 x 6″ filters and also have a label so you can quickly identify and retrieve each one without having to fumble through the entire pouch.