I'm tempted to go back to The Magnificent Seven on this one. Out of the 7 plus Ramirez (played by Eli Wallach) the only two who are slightly sub-par are possibly Brad Dexter and Horst Bucholtz both of whom did perfectly serviceable jobs....BUT for pretty much the most amazing cast ever:
* Albert Finney as Hercule Poirot * Lauren Bacall as Mrs. Harriet Belinda Hubbard * Sean Connery as Colonel Arbuthnott * Ingrid Bergman as Greta Ohlsson * Michael York as Count Rudolf Andrenyi * Vanessa Redgrave as Mary Debenham * Jacqueline Bisset as Countess Elena Andrenyi * Richard Widmark as Mr. Ratchett * John Gielgud as Edward Beddoes * Anthony Perkins as Hector McQueen * Martin Balsam as Bianchi * Rachel Roberts as Hildegarde Schmidt * Wendy Hiller as Princess Dragomiroff * Denis Quilley as Antonio (Tony) Foscarelli' * Colin Blakely as Cyrus B. "Dick" Hardman * Jean-Pierre Cassel as Pierre Michel * George Coulouris as Dr. Constantine