This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Old Yeller
A decision that no dog-owner should ever be forced to make.
Old Yeller's death at the hands of his owner is considered to be one of the saddest moments in cinema, and with good reason. So much so that the death in Old Yeller is often mentioned as "one of the few times it's okay for a man to cry".