What is the best polarizer to use for landscape?

[Dooglla]

I was looking through some of the pictures and was wondering which polarizer is best to use taking landscaper photography with a 20D and Canon 17-40L? A newbie with questions. 8)

[scottbergerphoto]

DSLR's need a circular polarizer for the AF to work. I use a B&W. Don't go cheap. You get what you pay for.
Scott