I read the sticky and several posts, so I know I need a slim, multi-coated, quality CP filter for my 17-40 f/4. I also know that I'll need one.
I have narrowed the election to the following which are the only alternatives in B&H:
1. B+W Multi-Coated (MC) Slim $ 164.95 B+W
2. B+W Kaeseman Slim $ 175 Kaesaman
3. Heliopan Multi-Coated (SH-PMC) Slim $ 178 Heliopan
4. Hoya CP/UV Comb (HMC) Multi-Coated - Ultra Thin $ 172.50 Hoya Ultra Thin (included just for completeness)
5. Hoya CPolarizer (S-HMC) Super Multi-Coated - Thin $ 172.50 Hoya Thin
All are from B&H (links included if required), non-threaded front (Heliopan does not include cap), Schott glass, multicoated (except for Kaesaman although it is encased).
Comments: a) the obvious choice should have been the B+W. However, it is not the MRC as recommended in posts and sticky and, b) the Hoya Ultra Thin is a CP/UV combo that could mean greater light falloff (even if B&H specifies the same 1.3 f-stop as the rest).