What is the main conflict of The Hound of the Baskervilles?
Conflicts in fiction are usually expressed in the pithy terms of "man vs. __________," where you can fill in the blank to express the conflict. Common conflicts include man vs. nature, man vs. man, man vs. society, man vs. God or Fate, man vs. himself, and man vs. technology. (Of course, "man" in this context means "human" and includes women.) The first "man" in the expression is the protagonist, hero, or main character of the story. The term that comes after the "vs." is the primary force that stands in the way of the protagonist achieving his goal or solving his problem.
Thinking of the conflict in The Hound of the Baskervilles in these terms, we can say that the conflict is man vs. man. The first man is Sherlock Holmes. Although Holmes's client, Sir Henry Baskerville, and Holmes's sidekick, Watson, are important characters and are on the same side as Holmes, the novella is written in such a way that Holmes is the central figure (even though he is absent for several chapters). Holmes's goal in the story is to solve the mystery of Sir Charles's death and to prevent any mischief from being done to Sir Henry. The primary impediment to that goal is a man, namely Mr. Stapleton. Stapleton has plotted for years to acquire the Baskerville estate, and his escapades with the fearsome hound resulted in Sir Charles's death and endanger Sir Henry. Using his amazing powers of deduction (and in spite some questionable methodology), Holmes reveals Stapleton's plot and secures Henry's life and estate.
In this intriguing Sherlock Holmes mystery, Holmes sets his wits against the crafty and sinister Stapleton and prevails. The conflict is man vs. man, specifically, Holmes vs. Stapleton.
The main conflict in this story is between the Baskervilles and the curse that seemingly haunts them. The danger to Sir Henry from the scheming Stapleton is real enough. Holmes and Watson, of course, champion Sir Henry and so they are on the side of good against the evil represented by Stapleton.
Stapleton's hounding (if one may forgive the pun) of first Sir Charles Baskerville and then Sir Henry stems from a family conflict, as it turns out. Stapleton, being a distant cousin of Sir Henry, has his eye on securing the Baskerville estate for himself.
This conflict in this story takes on an interesting extra dimension: the opposition between rationalism and superstition. The hound inspires all the more fear as it is believed to be a demon - as elaborated by the lurid legend related by Dr Mortimer early in the story. Holmes therefore has to deal with not just the physical threat to the Baskerville heir, he also has to debunk all the fear surrounding the hound, and the gloomy desolate moor and the forbidding Grimpen Mire. His scientific rationalism triumphs in the end, so that he finally remark with some confidence that 'I do not know that this place contains any secret which we have not already fathomed' (chapter 14).