The first, and arguably best outing for the ultimate Holmes/Watson outfit; Basil Rathbone and Nigel Bruce. Holmes stories are always better when they feature supernatural elements (all though they're always exposed as fake), and this is one of the few Rathbone stories set in Victorian times (when the series moved for Fox to Universal, the settings were brought foward to modern day, and often spoilt by war-time propaganda).
The famous story concerns the Baskerville curse, which sees heirs to Baskerville Hall being killed off by a vicious beast. Can Holmes save the last of the Baskerville's, Sir Henry (Nigel Green), or will the Curse be completed?
It always baffles me why this is the most filmed of Holmes' adventures, afterall the storyline dictates that he isn't even in it for a good portion of the film, which is a shame, as Rathbone is perfect as the eccentric sleuth, as is Bruce as the scatter-brained but endearing Watson.
There's a good supporting cast of horror veterans too; John Carradine as the butler and Lionel Atwill as an occult doctor, as well as Mary Gordon who played Holmes' maid Mrs. Hudson for the entire series.
Followed by 13 sequels (starting with 'The Adventures of Sherlock Holmes') and countless remakes (including one from Hammer).