“Elementary, my dear Watson!” Everyone has heard this, and most Sherlockians know the phrase was never spoken by the Master in The Canon. Holmes did describe things as “elementary” a number of times, and addressed Watson as “my dear Watson” frequently, but the terms were never combined the way popular culture would have us believe.
So did Holmes have a favorite phrase? Of course to answer that you would have to specify a length. If it was three words, “My dear Watson” might take the prize. For something longer than three words, I suggest “Possess our souls in patience”. Holmes says this in VALL, WIST, LADY, and 3GAR. (The Valley of Fear, The Adventure of Wisteria Lodge, The Disappearance of Lady Frances Carfax, and The Adventure of the Three Garridebs)
Was this really a Holmes original, or did he pick it up somewhere else? I think it sounds Biblical, but I do not know the context. After all, the editions of the Holy Writ are so numerous . . .
Here is the same phrase used in a May 27, 1918 newspaper article from New York Times, quoting a British General regarding the outcome of a WWI military operation. Maybe General Maurice was a Sherlockian, too!