which filter would you get for 135mm F/2L?

B+W 72mm MRC UV BRASS Filter F-PRO Multi-Coated
or
NEW HOYA 72mm HD High Definition Digital UV Filter

I would hate to ruin such great glass with a filter... Go naked

None. Maybe a CPL as I only have one to fit 77MM.

None.

Go with the B+W, I have them on
all my lenses.

I want to keep this lens for a long time and would hate to see dust get inside the glass

B+W CPL only.

I have on all of my lenses a B+W MRC UV-filter.

I'm in the no filter crowd.

I'd go with either. I use B+W only for consistency, but if your brand preference otherwise is for Hoya, the HD's supposed to be very good too.

i have some B+W filters as well but the Hoya HD spec does look good.

B+W, but only when the situation really, really calls for the extra protection (e.g. dusty environments with lots of wind, around rough salt water conditions). If you seldom find yourself in these situations, I'd save the money and put into a good circular polarizer. The coatings on these lenses are nowhere near as delicate as we've been made to believe. I've got 30 year old FD lenses that have never been UV covered, and optically look as though they were made yesterday.

Good call on the CPL I may buy that instead but it needs to be taken off for indoor or night photography right?

Yes; you should only use a polarizer when you need the specific effects (reduction of glare or reflection) that a polarizer can help with. Otherwise you're just throwing away a stop or two of light.

Jon wrote in post #10231521
Yes; you should only use a polarizer when you need the specific effects (reduction of glare or reflection) that a polarizer can help with. Otherwise you're just throwing away a stop or two of light.

also works as an ND filter on the sunny days when you want to shoot as wide as possible. I'm not sure what the OP will be shooting with this lens, but outdoor portraiture, weddings, etc would tend to have a need for a CPL.