The Pink Panther or Cheaper by the Dozen or It's Complicated, and that reason is Roxanne. He wrote and starred in this 1987 re-imagining of Rostand's Cryano de Bergerac, and he came up with perhaps the single finest romantic comedy since Annie Hall. The tone is whimsical, rhapsodic, without ever coming of as sappy or precious; Martin, with a comically oversized nose, has never been as likable or charming in a film. His script is an absolute gem--it's so beautifully written, even Daryl Hannah comes off well--and is both lushly romantic and slam-bang funny (a combination seen with rapidly decreasing frequency in the years since). I saw it in the theater in its original release; my VHS copy was, to put it mildly, well-worn. But I never miss a chance to take another look at Martin's masterpiece.
Watch it here for free. Crackle won't let me embed, the bastards.