Geordi La Forge as John H. Watson.
In the fictional stories of Sherlock Holmes, written by Sir Arthur Conan Doyle, Doctor John H. Watson serves as Holmes' assistant. His primary duty is to record the actions of Holmes for later oublication.
In 2365, Geordi La Forge invited Data to take part in a holodeck recreation of a Holmes novel. La Forge took on the role of Dr. Watson, while Data played Sherlock Holmes. (TNG: "Elementary, Dear Data")
La Forge reprised the role several years later, when the two officers were playing their respective parts again in 2369. (TNG: "Ship in a Bottle")