"When a doctor does go wrong he is the first of criminals. He has nerve and he has knowledge."
-- Sherlock Holmes, "The Adventure of the Speckled Band"
The interesting thing about this familiar quote is that it is so little supported in the Canon. The only "first of criminals" who is a physician in the Sherlock Holmes stories is the one who inspired the observation, the infamous Dr. Grimesby Roylott.
Dr. Ferrier in "The Naval Treaty" is, well, a doctor. Dr. Percy Trevelyan (great name!) of "The Resident Patient," is a client, as is The Hound of the Baskerville's Dr. James Mortimer.
A Dr. Wood is a minor character in The Valley of Fear. Dr. Leon Sterndale, apparently a physician as well as a big-game hunter, is one of a number of sympathetic revenge killers in the Canon and is given a free pass by Holmes in "The Adventure of the Devil's Foot."
Another physician, Dr. Ray Ernst, is a victim in "The Adventure of the Retired Colourman."
Only in "The Adventure of the Missing Three-Quarter" do we find a doctor who might conceivably be "first of criminals."
"Dr. Leslie Armstrong is certainly a man of energy and character," says Holmes. "I have not seen a man who, if he turns his talents that way, was more calculated to fill the gap left by the illustrious Moriarty."
Fortunately, however, he did not turn his talents that way and he wrings Holmes' hand in the end.
Who is your favorite doctor -- other than Dr. Watson! -- in the Canon?