there are alot of good polarizers out there. If you use an autofocus body, you should get a circular polarizer. You can buy linear polarizers, but those are usually only used in manual focus cameras as the makeup of the polarization lenses mess with autofocus in annoying ways. Tiffen makes some good ones, but the two best brands at the moment (IMO) are Hoya and B+W. If you buy a hoya, make sure it's their super HMC multicoated line or Pro1 line of filters. Those can get pretty pricey though, depending on the filter's size. A 77mm polarizer from either of those brands is usually around $120-$150.
I bought a B&W for the largest size I needed (77). The bought step up rings for my smaller lenses that I would use it on. Has worked great that way. And only had to buy 1 filter. Allowing me to get the best (some say).