Pentax SMC. B-W Multi-Resistant-Coated. Hoya Super-HM coated. Perhaps Heliopan, no experience with them myself but heard good things. Brass mount filter is nice, it won't "cold-weld" to the aluminium filter ring on lens (when you last saw a lens with brass filter ring? My Summicron has one, but that's quite an exception now...). Pentax Ghostless UV if you need 49mm size.
Good light!
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