Polar Pro Cinema series (Or Vivid series for polarized). They are built well, have no noticeable impact on image quality, and have better anti-reflective coatings to minimize the chance of a reflection ruining your footage. IMHO they are cheap enough that there is no reason not to buy some of the best, but YMMV. Another thing to consider is that in addition to softening the image, some cheap filters can introduce a color cast to your images that can be difficult to correct. If going with an unknown brand, you also want to make sure their weight is such that they do not interfere with the gimbal operation.