221b Baker Street, London.
Apparently when the book was written the highest numbered building was number 85. Below is number 85. (I thought that was interesting)
I love these tiles in the Tube station. to give you scale there is a bench in the picture below.
Another bit of trivia, when the museum opened they had a bit of argy bargy with the Abbey Bank down the road as to who got the post for Sherlock Holmes. The bank actually employed someone to reply to all the letters where people didnt realise that Sherlock wasnt a real person. The letter now go to the museum.