My understanding is you need a circular polarizer for any auto focus lens, so circular is the right choice.
Canon sells a good 3-piece filter set, polarizer, skylight, and ND filter. The price may seem a tad high, and I can't verify that it is exactly worth the money valuewise, but remember it doesn't make sense to put a cheap filter in front of a great lens.
The filter quality needs to be as good or better than the lens to preserve the lens' capabilities, including both the glass and the coatings.
I learned decades ago in still photography, don't go cheap on filters.