I have had many pola filters.
normal Hoya, Hoya Pro 1, Hoya HRT, B+W, Hama...
and finally I changed to a Hoya HD when I bought the 5DIII (Hoya HD for L lenses).
Hoya HD are the most expensive, but are really the best.
They allow a lot of light to come through, while in the same time making the very nice effect (dark blue sky, "popping" colors..), are very easy to clean (even "dried" salty -sticky- water after boat-trip).
I also have UV filter, also tried many before going to Hoya HD UV. This one is surprising and so amazing: quite close to the polarizer - I have a nice dark blue sky (not as much, but good) - and when you look at, you can think it is a hole and not a glass. You "can't" see it. It allow so much light to pass through..
Also, they are nearly impossible to strike/stripe...
Hoya HD are a really really "high-end" good choice.