I use Singh Ray filters, and B + W, and Tiffen, and Lee, and Hoya. Singh Ray produces some incredible and very innovative filters. But for something as basic as a circular polarizer, I very much doubt you will find a difference in real life usage.
I will not lose sleep wondering which is "better". The only one which I might be extra cautious is Hoya. Although I have never used a Nikon, knowing their excellence is making lenses, I will not have the slightest hesitation in using Nikon CP.