Barbara Stanwyck & Clark Gable. Stanwyck is my favorite actress and the pros pro. Every director, castmate, crew member loved and respected her work ethic and talent. Clark Gable was a terrific actor and made scores of excellent films in his long career.
Robert Mitchum the ultimate "menefreghista" (one who simply doesn't give a damn) but in fact was a very dedicated and generous actor. As Howard Hawks pointed out, that it was all an act and he cared deeply about his work.