I'm a huge fan of Cary Grant, and I think he's at his funniest in this film. In this scene he discovers a body at his aunts' house, and his reaction (which is probably the best double-take in the history of cinema) is absolutely priceless. I also really like Grant's various horrified reactions as he questions his aunts, particularly at around the five minute mark. This is probably a long-shot to make it out of the queue, but I have to give it a shot since it's such a classic scene.