Arthur Conan Doyle was a Scottish writer and physician who created the character Sherlock Holmes in 1887.

Sherlock Holmes first appeared in the novel A Study in Scarlet, which was published in 1887.

Dr. Watson is Sherlock Holmes's trusted companion and assistant. He is a retired army doctor who shares Holmes's Baker Street lodgings.

Professor Moriarty is Sherlock Holmes's arch-nemesis. He is a criminal mastermind and the head of a vast criminal organization.

The Murder of Roger Ackroyd is a novel by Agatha Christie, not Arthur Conan Doyle. It features Hercule Poirot, not Sherlock Holmes.

Agatha Christie was an English writer who created the character Hercule Poirot in 1920.

Hercule Poirot first appeared in the novel The Mysterious Affair at Styles, which was published in 1920.

Captain Hastings is Hercule Poirot's close friend and confidante. He is a retired army officer who often assists Poirot in his investigations.

The Hound of the Baskervilles is a Sherlock Holmes story, not a Hercule Poirot story.

Robert Downey Jr., Benedict Cumberbatch, and Jeremy Brett have all played Sherlock Holmes in film or television adaptations.

David Suchet, Kenneth Branagh, and Peter Ustinov have all played Hercule Poirot in film or television adaptations.

The use of deductive reasoning, the investigation of mysterious crimes, and the battle of wits between Holmes and his adversaries are all common themes in Sherlock Holmes stories.

The use of psychological insight, the investigation of seemingly impossible crimes, and the unmasking of hidden identities are all common themes in Hercule Poirot stories.

Holmes is a private detective, while Poirot is a police officer. Holmes is known for his eccentric personality, while Poirot is more reserved. Holmes relies on deductive reasoning, while Poirot uses psychological insight.

Sherlock Holmes and Hercule Poirot have inspired numerous adaptations in film, television, and other media. They have influenced the development of the detective fiction genre. They have become cultural icons and household names.