Zero Effect ousted itself to me as as a modern take on Sherlock Holmes (and had been my undisputed favorite modern adaptation until the new BBC series) with the opening and closing lines of the movie- Arlo says to a prospective client that "...He doesn't negotiate his fee. He works at a flat rate. Under some unusual circumstances, he'll work pro bono - never in between." and in The Problem of Thor Bridge Holmes says "My professional charges are upon a fixed scale, I do not vary them, save when I remit them altogether."
In the opening lines of A Scandal in Bohemia, Watson writes, "To Sherlock Holmes she is always the woman. I have seldom heard him mention her under any other name. In his eyes she eclipses and predominates the whole of her sex...there was but one woman to him, and that woman was the late Irene Adler, of dubious and questionable memory." The closing lines of Zero's report on the case he says "Perhaps the most able blackmailer of her time, she was at once the worthiest opponent and the greatest ally, and the only woman I have ever... the only woman, period."
Zero also says, in a seeming nod and wink to Watson himself, that though he tried to get Arlo to write of his exploits, he has refused to do so, and so has taken it upon himself to write the book.
EDIT: Upon close examination, I saw a couple other linguistic similarities in lines between Zero Effect and Sherlock Holmes; again, in Arlo's description of Zero in the beginning of the film he says of Daryl, "When private investigation won its most worthy champion, academia and the arts suffered a loss- a great loss." And in A Scandal in Bohemia, Watson writes "The stage lost a fine actor, even as science lost an acute reasoner, when he became a specialist in crime."
Zero says in in voice-over of his book-in-progress, "Passion is the enemy of precision." And Watson writes, also in the opening paragraph of A Scandal in Bohemia, "All emotions, and [love] particularly, were abhorrent to his cold, precise but admirably balanced mind...Grit in a sensitive instrument, or a crack in one of his high-power lenses, would not be more disturbing than a strong emotion in a nature such as his."
Also, at the end, Arlo leaves Zero to get married, which Watson also did to Holmes, although at the conclusion of The Sign of Four and a little time prior to the events of A Scandal in Bohemia.