According this fascinating piece in The Smithsonian, Franco Zeffirelli's 1979 weepfest The Champ is the most consistently effective tearjerker in the history of film. It's also the tearjerker most often used in scientific studies of grief and sadness:
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).
Sheerly Avni is a San Francisco-based arts and culture writer. Her work has appeared in Salon, LA Weekly, Mother Jones, and many other publications. You can follow her on twitter at @sheerly.