According to Smithsonian (hat tip, Vulture) scientists have found in laboratory situations that The Champ -- the bathetic 1979 remake with Ricky Schroder, not the fine 1931 original with Jackie Cooper -- is the most effective producer of tears. Is that the saddest they've got?
According to the article: 

"The Champ has been used in experiments to see if depressed people are more likely to cry than non-depressed people (they aren’t). It has helped determine whether people are more likely to spend money when they are sad (they are) and whether older people are more sensitive to grief than younger people (older people did report more sadness when they watched the scene). Dutch scientists used the scene when they studied the effect of sadness on people with binge eating disorders (sadness didn’t increase eating)."

Which all goes to show that a bad movie is useful for something.

As I've noted before, I'm not sure that there's a universal tear-wringer. But I have anecdotal evidence that for American men, sports films involving terminal illness or death (Brian's Song, Bang the Drum Slowly, Pride of the Yankees) are more effective than raw onions in producing the tear response.

And I know that for anyone who's nursed an ailing relative through illness that Dark Victory, Hanging Up, Away from Her, Hanging Up, Iris, The Notebook, Shadowlands and Stepmom do the trick.

The death of the family pet (The Biscuit Eater, Marley and Me, Old Yeller, Sounder) always works for me. As does loss of/reunion with parent or child (Antwone Fisher, Forrest Gump, To Each His Own, your favorite movie here________).

The Smithsonian article also cites All Mine to Give -- a very fine 1958 film about immigrants who die and whose eldest child has to parcel out his siblings to other families (on Christmas Eve) -- as a reliable tear inducer.

Tell me why you cry in movies.