The ones heavily featuring poignancy, really. Sad ones for sad's sake tend to feel fake to me, but those that contain the sweet sadness factor will have me blubbering rather shamelessly. Heck, the McDonald's commercial with the kid bringing his dad in for show and tell because he's the hero that gets him a Happy Meal will get me going if I'm not careful.
But the question is what movie, so I should be honest and pick one. I don't watch it any longer, because of this, but Old Yeller does it to me, every danged time. And not just when the kid has to shoot the dog, that's just the sad, though a real sad, but then the puppy at the end runs up, and you get the poignant, and break out the tissues, I'm done.