I've never rated Cary Grant as one of the great actors. My preference were for his more serious roles. So I'd have to go with Suspicion, North by North West and my fave To Catch a Thief with the lovely Grace Kelly.
My FAVORITE role was Operation Petticoat
Charade was great since he did not often have a really great actress and a great script at the same time. To Catch a Thief - the scene and setting made that movie. I always somehow thought Grant was a little detached in the movie.