Hi Zach,
First question, what do you want to shoot with your filters?
A quick rundown:
- Circular Polarizing Filter (CPL) can be used as both a neutral density filter (1-1.7 stops) but is most commonly used to minimize polarized light which can deepen blue skies, saturate colors and reduce things like glare from water.
- Solid ND filters. 2-4 stop solid ND filters are commonly used to photograph with a shallow dof (wide apertures) in bright conditions. Think outdoor portrait photography. 6-10 stop solid ND filters are commonly used to blur water in waterfalls, at the base of pools, or to blur away moving objects/people.
- Graduated ND filters are used to minimize the dynamic range of a scene by shading a portion of the frame but leaving the other portion unaffected. Anything with a sky, especially sunrises/sunsets you can put the shaded portion of a graduated ND filter over the sky to shade it back and leave the land unshaded in order to get proper exposure.
- Tinted filters can change the color of a scene. I know people that shoot with warmer up filters and love them. These, as the name implies, adjust the color temperature of a scene toward the warmer side.
- Singh-ray sells CPLs that have a blue-gold tint to them.
So there is a wide variety depending upon what you want to do. It has been argued that many of these features can now be done in post. For example, blur water of a waterfall, simply take many photos of the waterfall and stack them. Graduated ND can be accomplished with a tool in LR, etc.
Personally, I enjoy getting these images in camera. And it could be my post processing skills, but I tend to not like the grad ND filter in LR if I push it past ~1 stop, whereas I enjoy 2-3 stops with my optical ND filters. Also, post can't fix blown out highlights or too dark of shadows.
So, it depends upon what you want to do with filters. I tend to like Lee, Singh-Ray and B+W for brands. Most of my screw in filters are B+W (one Singh-Ray), and my grad ND filters are Lee, Singh Ray and Cokin.
Hope that helps.
Brant