I´m using a B+W Slim MRC and I´ve been happy with that. A more expensive alternative is the B+W XS-Pro MRC Nano, but I do not know what the extra dollars are providing.
Buy a filter with an good coating. A nano coating e.g. is easier to clean, and more scratch-resistant. And has (advertised) an better optical quality
Slim version if used with wideangle
Look for an high transmission filter (like from Heliopan), as they are not so dark. Lesser light is "swallowed" by the filter at the same polarisation effect, so the lens is faster
e.g.: Heliopan High Transmission Polfilter circ. SH-PMC