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Definitely Albert Finney as Hercule Poirot in Murder on the Orient Express, complete with fussy trimming with special sissors, exquisite waxing, and a hairnet to hold his mustache in place so it would not be mussed while sleeping.
Runner up would be Gregory Peck in The Gunfighter. Peck and his director agreed that he should have the look of photos of Old West gunmen to add to the film's realism. The studio heads had a fit when they saw the finished film because they thought women wouldn't come see leading man Peck if he had a mustache, but by then it was too late.