Sherlock Holmes is one of the most iconic and instantly recognized literary characters. Ironically, some of his appearance's most identifiable aspects—the cap, the pipe and so on—did not come from Sir Arthur Conan Doyle but from Doyle's illustrator, Sidney Paget, and stage actor William Gillette, who used them to make a memorable impression onstage. You can create a very recognizable Halloween costume as Holmes or try for something unique, if a little less easy to identify.
Three basic elements go into a classic Sherlock Holmes costume: the hat, the pipe and the coat. The hat is called a deerstalker or deerslayer, with flaps on the sides you can tie or untie around the ears. You can buy high-end wool models from formal hat shops or a simple "detective's hat" version from any costume shop. Any kind of pipe will do, but most presentations of Holmes had him with a Meerschaum pipe, with an outsized bowl. Meerschaum produces a type known as the mahogany calabash, which is billed as the "original Sherlock Holmes pipe." You can buy inexpensive plastic knockoffs at most costume shops if you don't want to spend a lot of money. Finally, the coat is known as an Inverness coat—popular among Victorian gentlemen at the time—and has a short cape. The coat should fit the deerstalker's pattern if possible; brown, tan or salt-and-pepper checkers are ideal.
The late Jeremy Brett portrayed Sherlock Holmes in a well-regarded British television series in the 1980s. It was notable in part because his Holmes often departed from the classic look, dispensing with the famous deerstalker and smoking a longer, thinner pipe. This costume will be harder to recognize than the classic look but will definitely be more distinct. Brett's appearance was distinctly funereal: a white Victorian shirt with a prominent collar, black vest, black pants and black coat. The pipe was tan and thin, about a foot long, and Brett's Holmes wore a black top hat. Black leather gloves completed the look. Brett would on occasion don the deerstalker and Inverness coat, which were straight gray herringbone, complemented by a matching vest.
Robert Downey Jr.
Robert Downey's performance as Holmes in the 2009 movie aimed for a rougher, more disheveled look. Instead of a deerstalker, he wore a black bowler derby, with a short dress coat in a deep, subtle purple (you can substitute black if you need to). A striped vest of dark colors, a light blue or white shirt, a neckcloth tie matching the coat, and black-and-white checkered pants (very small checks) round out the costume. Downey's pipe was very small and short. His clothes were often rumpled, and he usually wore a five o'clock shadow. You may want to refrain from shaving for a few days. To complete the look, don't iron the costume before you put it on.