This is actually a very interesting question!
Two of my all-time favorites are Greta Garbo and Myrna Loy. I am not sure that either one of them could make it in films today. Maybe Myrna could if she could shake the perfect wife image. But I am not to sure if Garbo could. Especially with how invasive media is today and how much of a private person she was.
My two favorite leading men are Clark Gable and Cary Grant. I think that Grant could make it today because he was able to play a wide variety of roles. However, Gable never seemed comfortable playing anything besides a macho