What do women want? A guy who's funny, sweet, thoughtful and who will wow them with a big romantic gesture after the inevitable third-act misunderstanding and break-up. (A nice smile and killer physique don't hurt either.) In other words, you've got to live up to the impossible standard set by these cinematic studs. Move over Matthew McConaughey and Paul Rudd; ahead the 20 best leading men under 40.