If you don’t already own an ND filter kit (or a variable ND) you need to get one. If you’re really new to this – ND filters essentially are like putting sunglasses on your lens. They allow you to control the exposure of your image without having to adjust your shutter speed, iris, or ISO setting.
Next to an ND filter, the polarizer is arguably the most important creative filter that every DP should have. Polarizers effectively cut out glare, reflections, and other unpleasant sources of light without darkening the entire image substantially.
With today’s cameras being as overly sharp as they are, there has never been a better time to own a diffusion filter. As the name implies, diffusion filters soften your subject slightly and can often create a beautiful but very subtle glow around certain parts of of the image.