What's your favourite type of filter, /p/?
I don't have any filters at the moment, but I'm interested in trying out a polarizing filter for sky backdrops and clearing up water, and maybe a graduated neutral density filter for girl in wheat field style pictures.
Please keep brand comparisons to a minimum. I think it's more interesting to talk about what a filter type enables you to do with a picture rather than which brand produces the best filter that will do that.