One of the best Poirot novels is 'Murder on the Orient Express'. It has a complex and captivating plot where Poirot has to solve a murder that occurs on a train filled with a diverse group of suspects. Another great one is 'Death on the Nile'. The setting on a Nile steamer adds an exotic touch to the mystery. 'The ABC Murders' is also excellent. It shows Poirot's brilliant mind as he tries to figure out the pattern behind a series of murders.
I think 'Cards on the Table' is among the best. It has an interesting premise where Poirot has to figure out who committed the murder during a game of bridge. 'Evil Under the Sun' is a great Poirot novel too. The beach setting and the web of secrets among the characters make it a really engaging read. And of course, 'The Mysterious Affair at Styles', which was the first Poirot novel, is a classic that introduced the character so well.
Well, 'Five Little Pigs' is a top - notch Poirot novel. Poirot has to solve a murder that happened years ago by interviewing the people involved, which is really fascinating. 'Hallowe'en Party' is also quite good. The spooky atmosphere of a Hallowe'en event combined with the mystery makes it stand out. 'Curtain' is another important one as it is Poirot's last case and gives a fitting end to the character's long career.