If you can find them in the sizes you need, Nikon makes about the best polarizers out there. I've tried B+W, Singh-Ray, Cokin, Tiffen, Hoya, and others but keep coming back to the Nikons. They're oversized so they don't vignette, they're truly neutral where some others have a slight cyan cast, and they are keyed such that they only rotate about 195 degrees in either direction. This little feature is especially nice when you're trying to remove one, as the thing doesn't just spin and spin. Finally, the Nikons are about 1/3 to 1/2 stop faster than anything else I've tried.
The Singh-Rays and Hoyas are also very good, and they make the more exotic warming polarizers. I personally didn't care for either the B+W or Tiffen polarizers as they exhibited a slight cyan cast.
You may consider it sacrilege to put anything Nikon on a Canon, but the Nikon polarizers are really good.
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