4. Irene Adler
A Scandal in Belgravia is not one of my favourite episodes of Sherlock. Unusually the story feels quite convoluted around the middle of the episode; however, Lara Pulver's Irene Adler is a real highlight. Whilst she could not be described as Sherlock's equal in the same way as some characters on this list, she is marvellously intelligent and makes a real impression on Sherlock. She is subtly reimagined from Conan Doyle's story as a dominatrix who extracts sensitive information from her clients and this suits the character perfectly. Her nakedness in the first scene encapsulates her intelligence. She shows all of herself to Sherlock but in doing so he cannot deduce anything about her (except the code to her safe!) Add to this the fact that she even manages to fake her own death and she is fully deserving of a place on this list. Could she even have inspired Sherlock to do the same?