The most notable of the short stories is Double Clue. It is the first appearance of Countess Vera Rossakoff (the closest thing to a romance with Poirot). She appears in twice more in The Big Four, and The Labours of Hercules.
Other favorites are Wasp's Nest in which Hercule Poirot comes to prevent a murder and leaves changing the murderer to be, The Lemesurier Inheritance where Poirot investigates the reality of family curse, The Chocolate Box, Poirot's only failure and The Third Floor Flat in which a crime happens in Poirot's own building.
Agatha's short stories are as good as her novels and are fun to read. These stories include characters Captain Hastings, Miss Lemon, Inspector Japp (or Chief Inspector Japp as he later beomes), and Countess Vera Rossakoff.