My most-used filter is the medium or #8 filter. If I want more contrast or separation, a #12 yellow is useful. Orange filters darken foliage and water too much in many cases for my taste. A light yellow-green is good on occasion, and, yes, I have a red filter, which doesn't get a lot of use any more. Polarizing is a must for color and black-and-white. I have a set of 55mm for my 35mm kit, a set of 67mm for my MF kit, and a set of 77mm for my LF kit. The 35mm and LF set are Tiffen, the MF kit is B&W. I figured my Hasselblad lenses deserved them. I would have bought one large size to fit all with adapters, but I acquired each setup at different times, starting small and moving up.