I love the works of Agatha Christie and, consequently, most of her film adaptations. Agatha Christie's Poirot is one of the best adaptations of lady Agatha, if not the best. As for "The ABC Murders," it's a fantastic episode, incredibly dark and one of the best in the series.
There's a lot of what we love about Poirot and Agatha Christie. It's not as good in its season as "Death in the Clouds," but it's still just fine and more. David Suchet is great as always, Hugh Fraser and Philip Jackson provide great support. Donald Douglas looks simply brilliant, cold and menacing, like Clarke, although this is not the same character as in the book. But the diamond of the episode is Donald Sumpter, who is just great as A. B. Cast, a man who is smart, humble, indecisive and influenced. The atmosphere of the episode is amazing, with a dark intensity and the music is perfect. The denouement is one of the most unexpected and striking in the show and it explains everything very logically and cunningly.
In conclusion, one of the best episodes of "Poirot" with one of the strongest acting performances for the series. 10/10